diff --git a/caps/DomainPolicy.cpp b/caps/DomainPolicy.cpp
index 2dcec2abe4b3..16029a2bdfdc 100644
--- a/caps/DomainPolicy.cpp
+++ b/caps/DomainPolicy.cpp
@@ -139,7 +139,7 @@ CopyURIs(const InfallibleTArray<URIParams>& aDomains, nsIDomainSet* aSet)
 }
 
 void
-DomainPolicy::ApplyClone(const DomainPolicyClone* aClone)
+DomainPolicy::ApplyClone(DomainPolicyClone* aClone)
 {
     CopyURIs(aClone->blacklist(), mBlacklist);
     CopyURIs(aClone->whitelist(), mWhitelist);
diff --git a/caps/nsIDomainPolicy.idl b/caps/nsIDomainPolicy.idl
index 3fd4020a60d0..54b0de4dbf98 100644
--- a/caps/nsIDomainPolicy.idl
+++ b/caps/nsIDomainPolicy.idl
@@ -17,7 +17,6 @@ class DomainPolicyClone;
 %}
 
 [ptr] native DomainPolicyClonePtr(mozilla::dom::DomainPolicyClone);
-[ptr] native DomainPolicyCloneConstPtr(const mozilla::dom::DomainPolicyClone);
 
 /*
  * When a domain policy is instantiated by invoking activateDomainPolicy() on
@@ -42,7 +41,7 @@ interface nsIDomainPolicy : nsISupports
     void deactivate();
 
     [noscript, notxpcom] void cloneDomainPolicy(in DomainPolicyClonePtr aClone);
-    [noscript, notxpcom] void applyClone(in DomainPolicyCloneConstPtr aClone);
+    [noscript, notxpcom] void applyClone(in DomainPolicyClonePtr aClone);
 };
 
 [scriptable, builtinclass, uuid(665c981b-0a0f-4229-ac06-a826e02d4f69)]
diff --git a/dom/ipc/ContentChild.cpp b/dom/ipc/ContentChild.cpp
index e271e10bb2cc..aee51eeee77a 100644
--- a/dom/ipc/ContentChild.cpp
+++ b/dom/ipc/ContentChild.cpp
@@ -502,24 +502,10 @@ NS_INTERFACE_MAP_BEGIN(ContentChild)
   NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIContentChild)
 NS_INTERFACE_MAP_END
 
-
-mozilla::ipc::IPCResult
-ContentChild::RecvSetXPCOMProcessAttributes(const XPCOMInitData& aXPCOMInit,
-                                            const StructuredCloneData& aInitialData,
-                                            nsTArray<LookAndFeelInt>&& aLookAndFeelIntCache)
-{
-  mLookAndFeelCache = aLookAndFeelIntCache;
-  InitXPCOM(aXPCOMInit, aInitialData);
-  InitGraphicsDeviceData();
-  return IPC_OK();
-}
-
 bool
 ContentChild::Init(MessageLoop* aIOLoop,
                    base::ProcessId aParentPid,
-                   IPC::Channel* aChannel,
-                   uint64_t aChildID,
-                   bool aIsForBrowser)
+                   IPC::Channel* aChannel)
 {
 #ifdef MOZ_WIDGET_GTK
   // We need to pass a display down to gtk_init because it's not going to
@@ -585,8 +571,7 @@ ContentChild::Init(MessageLoop* aIOLoop,
                                 XRE_GetProcessType());
 #endif
 
-  mID = aChildID;
-  mIsForBrowser = aIsForBrowser;
+  SendGetProcessAttributes(&mID, &mIsForBrowser);
 
 #ifdef NS_PRINTING
   // Force the creation of the nsPrintingProxy so that it's IPC counterpart,
@@ -948,14 +933,8 @@ ContentChild::InitGraphicsDeviceData()
 }
 
 void
-ContentChild::InitXPCOM(const XPCOMInitData& aXPCOMInit,
-                        const mozilla::dom::ipc::StructuredCloneData& aInitialData)
+ContentChild::InitXPCOM()
 {
-  SET_PREF_PHASE(pref_initPhase::BEGIN_ALL_PREFS);
-  for (unsigned int i = 0; i < aXPCOMInit.prefs().Length(); i++) {
-    Preferences::SetPreference(aXPCOMInit.prefs().ElementAt(i));
-  }
-  SET_PREF_PHASE(pref_initPhase::END_ALL_PREFS);
   // Do this as early as possible to get the parent process to initialize the
   // background thread since we'll likely need database information very soon.
   BackgroundChild::Startup();
@@ -978,29 +957,42 @@ ContentChild::InitXPCOM(const XPCOMInitData& aXPCOMInit,
   if (NS_FAILED(svc->RegisterListener(mConsoleListener)))
     NS_WARNING("Couldn't register console listener for child process");
 
-  mAvailableDictionaries = aXPCOMInit.dictionaries();
+  bool isOffline, isLangRTL, haveBidiKeyboards;
+  bool isConnected;
+  int32_t captivePortalState;
+  ClipboardCapabilities clipboardCaps;
+  DomainPolicyClone domainPolicy;
+  StructuredCloneData initialData;
+  OptionalURIParams userContentSheetURL;
 
-  RecvSetOffline(aXPCOMInit.isOffline());
-  RecvSetConnectivity(aXPCOMInit.isConnected());
-  RecvSetCaptivePortalState(aXPCOMInit.captivePortalState());
-  RecvBidiKeyboardNotify(aXPCOMInit.isLangRTL(), aXPCOMInit.haveBidiKeyboards());
+  SendGetXPCOMProcessAttributes(&isOffline, &isConnected, &captivePortalState,
+                                &isLangRTL, &haveBidiKeyboards,
+                                &mAvailableDictionaries,
+                                &clipboardCaps, &domainPolicy, &initialData,
+                                &mFontFamilies, &userContentSheetURL,
+                                &mLookAndFeelCache);
+
+  RecvSetOffline(isOffline);
+  RecvSetConnectivity(isConnected);
+  RecvSetCaptivePortalState(captivePortalState);
+  RecvBidiKeyboardNotify(isLangRTL, haveBidiKeyboards);
 
   // Create the CPOW manager as soon as possible.
   SendPJavaScriptConstructor();
 
-  if (aXPCOMInit.domainPolicy().active()) {
+  if (domainPolicy.active()) {
     nsIScriptSecurityManager* ssm = nsContentUtils::GetSecurityManager();
     MOZ_ASSERT(ssm);
     ssm->ActivateDomainPolicyInternal(getter_AddRefs(mPolicy));
     if (!mPolicy) {
       MOZ_CRASH("Failed to activate domain policy.");
     }
-    mPolicy->ApplyClone(&aXPCOMInit.domainPolicy());
+    mPolicy->ApplyClone(&domainPolicy);
   }
 
   nsCOMPtr<nsIClipboard> clipboard(do_GetService("@mozilla.org/widget/clipboard;1"));
   if (nsCOMPtr<nsIClipboardProxy> clipboardProxy = do_QueryInterface(clipboard)) {
-    clipboardProxy->SetCapabilities(aXPCOMInit.clipboardCaps());
+    clipboardProxy->SetCapabilities(clipboardCaps);
   }
 
   {
@@ -1010,9 +1002,7 @@ ContentChild::InitXPCOM(const XPCOMInitData& aXPCOMInit,
     }
     ErrorResult rv;
     JS::RootedValue data(jsapi.cx());
-    mozilla::dom::ipc::StructuredCloneData id;
-    id.Copy(aInitialData);
-    id.Read(jsapi.cx(), &data, rv);
+    initialData.Read(jsapi.cx(), &data, rv);
     if (NS_WARN_IF(rv.Failed())) {
       MOZ_CRASH();
     }
@@ -1021,7 +1011,7 @@ ContentChild::InitXPCOM(const XPCOMInitData& aXPCOMInit,
   }
 
   // The stylesheet cache is not ready yet. Store this URL for future use.
-  nsCOMPtr<nsIURI> ucsURL = DeserializeURI(aXPCOMInit.userContentSheetURL());
+  nsCOMPtr<nsIURI> ucsURL = DeserializeURI(userContentSheetURL);
   nsLayoutStylesheetCache::SetUserContentCSSURL(ucsURL);
 
   // This will register cross-process observer.
diff --git a/dom/ipc/ContentChild.h b/dom/ipc/ContentChild.h
index d712f5c8c165..c9e4153c2499 100644
--- a/dom/ipc/ContentChild.h
+++ b/dom/ipc/ContentChild.h
@@ -96,12 +96,9 @@ public:
 
   bool Init(MessageLoop* aIOLoop,
             base::ProcessId aParentPid,
-            IPC::Channel* aChannel,
-            uint64_t aChildID,
-            bool aIsForBrowser);
+            IPC::Channel* aChannel);
 
-  void InitXPCOM(const XPCOMInitData& aXPCOMInit,
-                 const mozilla::dom::ipc::StructuredCloneData& aInitialData);
+  void InitXPCOM();
 
   void InitGraphicsDeviceData();
 
@@ -576,11 +573,6 @@ public:
           const bool& minimizeMemoryUsage,
           const MaybeFileDesc& DMDFile) override;
 
-  virtual mozilla::ipc::IPCResult
-  RecvSetXPCOMProcessAttributes(const XPCOMInitData& aXPCOMInit,
-                                const StructuredCloneData& aInitialData,
-                                nsTArray<LookAndFeelInt>&& aLookAndFeelIntCache) override;
-
 #if defined(XP_WIN) && defined(ACCESSIBILITY)
   bool
   SendGetA11yContentId();
diff --git a/dom/ipc/ContentParent.cpp b/dom/ipc/ContentParent.cpp
index 7db9e85399f7..030d413fd003 100644
--- a/dom/ipc/ContentParent.cpp
+++ b/dom/ipc/ContentParent.cpp
@@ -186,8 +186,6 @@
 
 #include "nsLayoutStylesheetCache.h"
 
-#include "ContentPrefs.h"
-
 #ifdef MOZ_WEBRTC
 #include "signaling/src/peerconnection/WebrtcGlobalParent.h"
 #endif
@@ -1826,48 +1824,6 @@ ContentParent::LaunchSubprocess(ProcessPriority aInitialPriority /* = PROCESS_PR
   PROFILER_LABEL_FUNC(js::ProfileEntry::Category::OTHER);
 
   std::vector<std::string> extraArgs;
-  extraArgs.push_back("-childID");
-  char idStr[21];
-  SprintfLiteral(idStr, "%" PRId64, static_cast<uint64_t>(mChildID));
-  extraArgs.push_back(idStr);
-  extraArgs.push_back(IsForBrowser() ? "-isForBrowser" : "-notForBrowser");
-
-  std::stringstream boolPrefs;
-  std::stringstream intPrefs;
-  std::stringstream stringPrefs;
-
-  size_t prefsLen;
-  ContentPrefs::GetContentPrefs(&prefsLen);
-
-  for (unsigned int i = 0; i < prefsLen; i++) {
-    MOZ_ASSERT(i == 0 || strcmp(ContentPrefs::GetContentPref(i), ContentPrefs::GetContentPref(i - 1)) > 0);
-    switch (Preferences::GetType(ContentPrefs::GetContentPref(i))) {
-    case nsIPrefBranch::PREF_INT:
-      intPrefs << i << ':' << Preferences::GetInt(ContentPrefs::GetContentPref(i)) << '|';
-      break;
-    case nsIPrefBranch::PREF_BOOL:
-      boolPrefs << i << ':' << Preferences::GetBool(ContentPrefs::GetContentPref(i)) << '|';
-      break;
-    case nsIPrefBranch::PREF_STRING: {
-      std::string value(Preferences::GetCString(ContentPrefs::GetContentPref(i)).get());
-      stringPrefs << i << ':' << value.length() << ':' << value << '|';
-    }
-      break;
-    case nsIPrefBranch::PREF_INVALID:
-      break;
-    default:
-      printf("preference type: %x\n", Preferences::GetType(ContentPrefs::GetContentPref(i)));
-      MOZ_CRASH();
-    }
-  }
-
-  extraArgs.push_back("-intPrefs");
-  extraArgs.push_back(intPrefs.str());
-  extraArgs.push_back("-boolPrefs");
-  extraArgs.push_back(boolPrefs.str());
-  extraArgs.push_back("-stringPrefs");
-  extraArgs.push_back(stringPrefs.str());
-
   if (!mSubprocess->LaunchAndWaitForProcessHandle(extraArgs)) {
     MarkAsDead();
     return false;
@@ -1887,17 +1843,6 @@ ContentParent::LaunchSubprocess(ProcessPriority aInitialPriority /* = PROCESS_PR
   // Set a reply timeout for CPOWs.
   SetReplyTimeoutMs(Preferences::GetInt("dom.ipc.cpow.timeout", 0));
 
-  Telemetry::Accumulate(Telemetry::CONTENT_PROCESS_LAUNCH_TIME_MS,
-                        static_cast<uint32_t>((TimeStamp::Now() - mLaunchTS)
-                                              .ToMilliseconds()));
-
-  nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
-  if (obs) {
-    nsAutoString cpId;
-    cpId.AppendInt(static_cast<uint64_t>(this->ChildID()));
-    obs->NotifyObservers(static_cast<nsIObserver*>(this), "ipc:content-initializing", cpId.get());
-  }
-
   return true;
 }
 
@@ -1955,90 +1900,6 @@ ContentParent::InitInternal(ProcessPriority aInitialPriority,
                             bool aSetupOffMainThreadCompositing,
                             bool aSendRegisteredChrome)
 {
-  Telemetry::Accumulate(Telemetry::CONTENT_PROCESS_LAUNCH_TIME_MS,
-                        static_cast<uint32_t>((TimeStamp::Now() - mLaunchTS)
-                                              .ToMilliseconds()));
-
-  XPCOMInitData xpcomInit;
-
-  Preferences::GetPreferences(&xpcomInit.prefs());
-  nsCOMPtr<nsIIOService> io(do_GetIOService());
-  MOZ_ASSERT(io, "No IO service?");
-  DebugOnly<nsresult> rv = io->GetOffline(&xpcomInit.isOffline());
-  MOZ_ASSERT(NS_SUCCEEDED(rv), "Failed getting offline?");
-
-  rv = io->GetConnectivity(&xpcomInit.isConnected());
-  MOZ_ASSERT(NS_SUCCEEDED(rv), "Failed getting connectivity?");
-
-  xpcomInit.captivePortalState() = nsICaptivePortalService::UNKNOWN;
-  nsCOMPtr<nsICaptivePortalService> cps = do_GetService(NS_CAPTIVEPORTAL_CONTRACTID);
-  if (cps) {
-    cps->GetState(&xpcomInit.captivePortalState());
-  }
-
-  nsIBidiKeyboard* bidi = nsContentUtils::GetBidiKeyboard();
-
-  xpcomInit.isLangRTL() = false;
-  xpcomInit.haveBidiKeyboards() = false;
-  if (bidi) {
-    bidi->IsLangRTL(&xpcomInit.isLangRTL());
-    bidi->GetHaveBidiKeyboards(&xpcomInit.haveBidiKeyboards());
-  }
-
-  nsCOMPtr<nsISpellChecker> spellChecker(do_GetService(NS_SPELLCHECKER_CONTRACTID));
-  MOZ_ASSERT(spellChecker, "No spell checker?");
-
-  spellChecker->GetDictionaryList(&xpcomInit.dictionaries());
-
-  nsCOMPtr<nsIClipboard> clipboard(do_GetService("@mozilla.org/widget/clipboard;1"));
-  MOZ_ASSERT(clipboard, "No clipboard?");
-
-  rv = clipboard->SupportsSelectionClipboard(&xpcomInit.clipboardCaps().supportsSelectionClipboard());
-  MOZ_ASSERT(NS_SUCCEEDED(rv));
-
-  rv = clipboard->SupportsFindClipboard(&xpcomInit.clipboardCaps().supportsFindClipboard());
-  MOZ_ASSERT(NS_SUCCEEDED(rv));
-
-  // Let's copy the domain policy from the parent to the child (if it's active).
-  StructuredCloneData initialData;
-  nsIScriptSecurityManager* ssm = nsContentUtils::GetSecurityManager();
-  if (ssm) {
-    ssm->CloneDomainPolicy(&xpcomInit.domainPolicy());
-
-    if (nsFrameMessageManager* mm = nsFrameMessageManager::sParentProcessManager) {
-      AutoJSAPI jsapi;
-      if (NS_WARN_IF(!jsapi.Init(xpc::PrivilegedJunkScope()))) {
-        MOZ_CRASH();
-      }
-      JS::RootedValue init(jsapi.cx());
-      nsresult result = mm->GetInitialProcessData(jsapi.cx(), &init);
-      if (NS_FAILED(result)) {
-        MOZ_CRASH();
-      }
-
-      ErrorResult rv;
-      initialData.Write(jsapi.cx(), init, rv);
-      if (NS_WARN_IF(rv.Failed())) {
-        rv.SuppressException();
-        MOZ_CRASH();
-      }
-    }
-  }
-  // This is only implemented (returns a non-empty list) by MacOSX at present.
-  gfxPlatform::GetPlatform()->GetSystemFontFamilyList(&xpcomInit.fontFamilies());
-  nsTArray<LookAndFeelInt> lnfCache = LookAndFeel::GetIntCache();
-
-  // Content processes have no permission to access profile directory, so we
-  // send the file URL instead.
-  StyleSheet* ucs = nsLayoutStylesheetCache::For(StyleBackendType::Gecko)->UserContentSheet();
-  if (ucs) {
-    SerializeURI(ucs->GetSheetURI(), xpcomInit.userContentSheetURL());
-  } else {
-    SerializeURI(nullptr, xpcomInit.userContentSheetURL());
-  }
-
-  Unused << SendSetXPCOMProcessAttributes(xpcomInit, initialData, lnfCache);
-
   if (aSendRegisteredChrome) {
     nsCOMPtr<nsIChromeRegistry> registrySvc = nsChromeRegistry::GetService();
     nsChromeRegistryChrome* chromeRegistry =
@@ -2210,6 +2071,13 @@ ContentParent::Pid() const
   return base::GetProcId(mSubprocess->GetChildProcessHandle());
 }
 
+mozilla::ipc::IPCResult
+ContentParent::RecvReadPrefsArray(InfallibleTArray<PrefSetting>* aPrefs)
+{
+  Preferences::GetPreferences(aPrefs);
+  return IPC_OK();
+}
+
 mozilla::ipc::IPCResult
 ContentParent::RecvGetGfxVars(InfallibleTArray<GfxVarUpdate>* aVars)
 {
@@ -2680,6 +2548,118 @@ ContentParent::RecvInitBackground(Endpoint<PBackgroundParent>&& aEndpoint)
   return IPC_OK();
 }
 
+mozilla::ipc::IPCResult
+ContentParent::RecvGetProcessAttributes(ContentParentId* aCpId,
+                                        bool* aIsForBrowser)
+{
+  *aCpId = mChildID;
+  *aIsForBrowser = mIsForBrowser;
+
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult
+ContentParent::RecvGetXPCOMProcessAttributes(bool* aIsOffline,
+                                             bool* aIsConnected,
+                                             int32_t* aCaptivePortalState,
+                                             bool* aIsLangRTL,
+                                             bool* aHaveBidiKeyboards,
+                                             InfallibleTArray<nsString>* dictionaries,
+                                             ClipboardCapabilities* clipboardCaps,
+                                             DomainPolicyClone* domainPolicy,
+                                             StructuredCloneData* aInitialData,
+                                             InfallibleTArray<FontFamilyListEntry>* fontFamilies,
+                                             OptionalURIParams* aUserContentCSSURL,
+                                             nsTArray<LookAndFeelInt>* aLookAndFeelIntCache)
+{
+  Telemetry::Accumulate(Telemetry::CONTENT_PROCESS_LAUNCH_TIME_MS,
+                        static_cast<uint32_t>((TimeStamp::Now() - mLaunchTS)
+                                              .ToMilliseconds()));
+
+  nsCOMPtr<nsIIOService> io(do_GetIOService());
+  MOZ_ASSERT(io, "No IO service?");
+  DebugOnly<nsresult> rv = io->GetOffline(aIsOffline);
+  MOZ_ASSERT(NS_SUCCEEDED(rv), "Failed getting offline?");
+
+  rv = io->GetConnectivity(aIsConnected);
+  MOZ_ASSERT(NS_SUCCEEDED(rv), "Failed getting connectivity?");
+
+  *aCaptivePortalState = nsICaptivePortalService::UNKNOWN;
+  nsCOMPtr<nsICaptivePortalService> cps = do_GetService(NS_CAPTIVEPORTAL_CONTRACTID);
+  if (cps) {
+    cps->GetState(aCaptivePortalState);
+  }
+
+  nsIBidiKeyboard* bidi = nsContentUtils::GetBidiKeyboard();
+
+  *aIsLangRTL = false;
+  *aHaveBidiKeyboards = false;
+  if (bidi) {
+    bidi->IsLangRTL(aIsLangRTL);
+    bidi->GetHaveBidiKeyboards(aHaveBidiKeyboards);
+  }
+
+  nsCOMPtr<nsISpellChecker> spellChecker(do_GetService(NS_SPELLCHECKER_CONTRACTID));
+  MOZ_ASSERT(spellChecker, "No spell checker?");
+
+  spellChecker->GetDictionaryList(dictionaries);
+
+  nsCOMPtr<nsIClipboard> clipboard(do_GetService("@mozilla.org/widget/clipboard;1"));
+  MOZ_ASSERT(clipboard, "No clipboard?");
+
+  rv = clipboard->SupportsSelectionClipboard(&clipboardCaps->supportsSelectionClipboard());
+  MOZ_ASSERT(NS_SUCCEEDED(rv));
+
+  rv = clipboard->SupportsFindClipboard(&clipboardCaps->supportsFindClipboard());
+  MOZ_ASSERT(NS_SUCCEEDED(rv));
+
+  // Let's copy the domain policy from the parent to the child (if it's active).
+  nsIScriptSecurityManager* ssm = nsContentUtils::GetSecurityManager();
+  NS_ENSURE_TRUE(ssm, IPC_OK());
+  ssm->CloneDomainPolicy(domainPolicy);
+
+  if (nsFrameMessageManager* mm = nsFrameMessageManager::sParentProcessManager) {
+    AutoJSAPI jsapi;
+    if (NS_WARN_IF(!jsapi.Init(xpc::PrivilegedJunkScope()))) {
+      return IPC_FAIL_NO_REASON(this);
+    }
+    JS::RootedValue init(jsapi.cx());
+    nsresult result = mm->GetInitialProcessData(jsapi.cx(), &init);
+    if (NS_FAILED(result)) {
+      return IPC_FAIL_NO_REASON(this);
+    }
+
+    ErrorResult rv;
+    aInitialData->Write(jsapi.cx(), init, rv);
+    if (NS_WARN_IF(rv.Failed())) {
+      rv.SuppressException();
+      return IPC_FAIL_NO_REASON(this);
+    }
+  }
+
+  // This is only implemented (returns a non-empty list) by MacOSX at present.
+  gfxPlatform::GetPlatform()->GetSystemFontFamilyList(fontFamilies);
+  *aLookAndFeelIntCache = LookAndFeel::GetIntCache();
+
+  // Content processes have no permission to access profile directory, so we
+  // send the file URL instead.
+  StyleSheet* ucs = nsLayoutStylesheetCache::For(StyleBackendType::Gecko)->UserContentSheet();
+  if (ucs) {
+    SerializeURI(ucs->GetSheetURI(), *aUserContentCSSURL);
+  } else {
+    SerializeURI(nullptr, *aUserContentCSSURL);
+  }
+
+  nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
+  if (obs) {
+    nsAutoString cpId;
+    cpId.AppendInt(static_cast<uint64_t>(this->ChildID()));
+    obs->NotifyObservers(static_cast<nsIObserver*>(this), "ipc:content-initializing", cpId.get());
+  }
+
+  return IPC_OK();
+}
+
 mozilla::jsipc::PJavaScriptParent *
 ContentParent::AllocPJavaScriptParent()
 {
diff --git a/dom/ipc/ContentParent.h b/dom/ipc/ContentParent.h
index 3c79fb559861..f0650a00c601 100644
--- a/dom/ipc/ContentParent.h
+++ b/dom/ipc/ContentParent.h
@@ -746,6 +746,24 @@ private:
 
   virtual mozilla::ipc::IPCResult RecvInitBackground(Endpoint<mozilla::ipc::PBackgroundParent>&& aEndpoint) override;
 
+  virtual mozilla::ipc::IPCResult RecvGetProcessAttributes(ContentParentId* aCpId,
+                                                           bool* aIsForBrowser) override;
+
+  virtual mozilla::ipc::IPCResult
+  RecvGetXPCOMProcessAttributes(bool* aIsOffline,
+                                bool* aIsConnected,
+                                int32_t* aCaptivePortalState,
+                                bool* aIsLangRTL,
+                                bool* aHaveBidiKeyboards,
+                                InfallibleTArray<nsString>* dictionaries,
+                                ClipboardCapabilities* clipboardCaps,
+                                DomainPolicyClone* domainPolicy,
+                                StructuredCloneData* initialData,
+                                InfallibleTArray<FontFamilyListEntry>* fontFamilies,
+                                OptionalURIParams* aUserContentSheetURL,
+                                nsTArray<LookAndFeelInt>* aLookAndFeelIntCache) override;
+
+
   mozilla::ipc::IPCResult RecvAddMemoryReport(const MemoryReport& aReport) override;
   mozilla::ipc::IPCResult RecvFinishMemoryReport(const uint32_t& aGeneration) override;
 
@@ -880,6 +898,7 @@ private:
   virtual bool
   DeallocPWebBrowserPersistDocumentParent(PWebBrowserPersistDocumentParent* aActor) override;
 
+  virtual mozilla::ipc::IPCResult RecvReadPrefsArray(InfallibleTArray<PrefSetting>* aPrefs) override;
   virtual mozilla::ipc::IPCResult RecvGetGfxVars(InfallibleTArray<GfxVarUpdate>* aVars) override;
 
   virtual mozilla::ipc::IPCResult RecvReadFontList(InfallibleTArray<FontListEntry>* retValue) override;
diff --git a/dom/ipc/ContentPrefs.cpp b/dom/ipc/ContentPrefs.cpp
deleted file mode 100644
index 343fe98d03f5..000000000000
--- a/dom/ipc/ContentPrefs.cpp
+++ /dev/null
@@ -1,275 +0,0 @@
-/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
-/* vim: set ts=8 sts=2 et sw=2 tw=80: */
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-#include "ContentPrefs.h"
-
-const char* mozilla::dom::ContentPrefs::gInitPrefs[] = {
-  "accessibility.monoaudio.enable",
-  "accessibility.mouse_focuses_formcontrol",
-  "accessibility.tabfocus_applies_to_xul",
-  "app.update.channel",
-  "browser.dom.window.dump.enabled",
-  "browser.sessionhistory.max_entries",
-  "browser.sessionhistory.max_total_viewers",
-  "content.cors.disable",
-  "content.cors.no_private_data",
-  "content.notify.backoffcount",
-  "content.notify.interval",
-  "content.notify.ontimer",
-  "content.sink.enable_perf_mode",
-  "content.sink.event_probe_rate",
-  "content.sink.initial_perf_time",
-  "content.sink.interactive_deflect_count",
-  "content.sink.interactive_parse_time",
-  "content.sink.interactive_time",
-  "content.sink.pending_event_mode",
-  "content.sink.perf_deflect_count",
-  "content.sink.perf_parse_time",
-  "device.storage.prompt.testing",
-  "device.storage.writable.name",
-  "dom.allow_XUL_XBL_for_file",
-  "dom.allow_cut_copy",
-  "dom.enable_frame_timing",
-  "dom.enable_performance",
-  "dom.enable_resource_timing",
-  "dom.event.handling-user-input-time-limit",
-  "dom.forms.autocomplete.experimental",
-  "dom.ipc.processPriorityManager.backgroundGracePeriodMS",
-  "dom.ipc.processPriorityManager.backgroundPerceivableGracePeriodMS",
-  "dom.max_chrome_script_run_time",
-  "dom.max_script_run_time",
-  "dom.performance.enable_notify_performance_timing",
-  "dom.performance.enable_user_timing_logging",
-  "dom.storage.testing",
-  "dom.url.encode_decode_hash",
-  "dom.url.getters_decode_hash",
-  "dom.use_watchdog",
-  "dom.vibrator.enabled",
-  "dom.vibrator.max_vibrate_list_len",
-  "dom.vibrator.max_vibrate_ms",
-  "focusmanager.testmode",
-  "font.size.inflation.disabledInMasterProcess",
-  "font.size.inflation.emPerLine",
-  "font.size.inflation.forceEnabled",
-  "font.size.inflation.lineThreshold",
-  "font.size.inflation.mappingIntercept",
-  "font.size.inflation.maxRatio",
-  "font.size.inflation.minTwips",
-  "full-screen-api.allow-trusted-requests-only",
-  "full-screen-api.enabled",
-  "full-screen-api.unprefix.enabled",
-  "gfx.font_rendering.opentype_svg.enabled",
-  "hangmonitor.timeout",
-  "html5.flushtimer.initialdelay",
-  "html5.flushtimer.subsequentdelay",
-  "html5.offmainthread",
-  "intl.charset.fallback.tld",
-  "intl.ime.hack.on_ime_unaware_apps.fire_key_events_for_composition",
-  "javascript.enabled",
-  "javascript.options.asmjs",
-  "javascript.options.asyncstack",
-  "javascript.options.baselinejit",
-  "javascript.options.baselinejit.threshold",
-  "javascript.options.baselinejit.unsafe_eager_compilation",
-  "javascript.options.discardSystemSource",
-  "javascript.options.dump_stack_on_debuggee_would_run",
-  "javascript.options.gczeal",
-  "javascript.options.gczeal.frequency",
-  "javascript.options.ion",
-  "javascript.options.ion.offthread_compilation",
-  "javascript.options.ion.threshold",
-  "javascript.options.ion.unsafe_eager_compilation",
-  "javascript.options.jit.full_debug_checks",
-  "javascript.options.native_regexp",
-  "javascript.options.parallel_parsing",
-  "javascript.options.shared_memory",
-  "javascript.options.strict",
-  "javascript.options.strict.debug",
-  "javascript.options.throw_on_asmjs_validation_failure",
-  "javascript.options.throw_on_debuggee_would_run",
-  "javascript.options.wasm",
-  "javascript.options.wasm_baselinejit",
-  "javascript.options.werror",
-  "javascript.use_us_english_locale",
-  "jsloader.reuseGlobal",
-  "layout.css.all-shorthand.enabled",
-  "layout.css.background-blend-mode.enabled",
-  "layout.css.background-clip-text.enabled",
-  "layout.css.box-decoration-break.enabled",
-  "layout.css.color-adjust.enabled",
-  "layout.css.contain.enabled",
-  "layout.css.control-characters.visible",
-  "layout.css.display-flow-root.enabled",
-  "layout.css.expensive-style-struct-assertions.enabled",
-  "layout.css.float-logical-values.enabled",
-  "layout.css.font-variations.enabled",
-  "layout.css.grid.enabled",
-  "layout.css.image-orientation.enabled",
-  "layout.css.initial-letter.enabled",
-  "layout.css.isolation.enabled",
-  "layout.css.mix-blend-mode.enabled",
-  "layout.css.object-fit-and-position.enabled",
-  "layout.css.osx-font-smoothing.enabled",
-  "layout.css.overflow-clip-box.enabled",
-  "layout.css.prefixes.animations",
-  "layout.css.prefixes.border-image",
-  "layout.css.prefixes.box-sizing",
-  "layout.css.prefixes.device-pixel-ratio-webkit",
-  "layout.css.prefixes.font-features",
-  "layout.css.prefixes.gradients",
-  "layout.css.prefixes.transforms",
-  "layout.css.prefixes.transitions",
-  "layout.css.prefixes.webkit",
-  "layout.css.scope-pseudo.enabled",
-  "layout.css.scroll-behavior.property-enabled",
-  "layout.css.scroll-snap.enabled",
-  "layout.css.shape-outside.enabled",
-  "layout.css.text-align-unsafe-value.enabled",
-  "layout.css.text-combine-upright-digits.enabled",
-  "layout.css.text-combine-upright.enabled",
-  "layout.css.touch_action.enabled",
-  "layout.css.unprefixing-service.enabled",
-  "layout.css.unprefixing-service.globally-whitelisted",
-  "layout.css.unprefixing-service.include-test-domains",
-  "layout.css.variables.enabled",
-  "layout.css.visited_links_enabled",
-  "layout.idle_period.required_quiescent_frames",
-  "layout.idle_period.time_limit",
-  "layout.interruptible-reflow.enabled",
-  "mathml.disabled",
-  "media.apple.forcevda",
-  "media.clearkey.persistent-license.enabled",
-  "media.cubeb_latency_msg_frames",
-  "media.cubeb_latency_playback_ms",
-  "media.decoder-doctor.wmf-disabled-is-failure",
-  "media.decoder.fuzzing.dont-delay-inputexhausted",
-  "media.decoder.fuzzing.enabled",
-  "media.decoder.fuzzing.video-output-minimum-interval-ms",
-  "media.decoder.limit",
-  "media.decoder.recycle.enabled",
-  "media.dormant-on-pause-timeout-ms",
-  "media.eme.audio.blank",
-  "media.eme.enabled",
-  "media.eme.video.blank",
-  "media.ffmpeg.enabled",
-  "media.ffvpx.enabled",
-  "media.flac.enabled",
-  "media.forcestereo.enabled",
-  "media.gmp.async-shutdown-timeout",
-  "media.gmp.decoder.aac",
-  "media.gmp.decoder.enabled",
-  "media.gmp.decoder.h264",
-  "media.gmp.insecure.allow",
-  "media.gpu-process-decoder",
-  "media.libavcodec.allow-obsolete",
-  "media.num-decode-threads",
-  "media.ogg.enabled",
-  "media.ogg.flac.enabled",
-  "media.resampling.enabled",
-  "media.resampling.rate",
-  "media.ruin-av-sync.enabled",
-  "media.rust.test_mode",
-  "media.suspend-bkgnd-video.delay-ms",
-  "media.suspend-bkgnd-video.enabled",
-  "media.use-blank-decoder",
-  "media.video_stats.enabled",
-  "media.volume_scale",
-  "media.webspeech.recognition.enable",
-  "media.webspeech.recognition.force_enable",
-  "media.webspeech.synth.force_global_queue",
-  "media.webspeech.test.enable",
-  "media.webspeech.test.fake_fsm_events",
-  "media.webspeech.test.fake_recognition_service",
-  "media.wmf.allow-unsupported-resolutions",
-  "media.wmf.decoder.thread-count",
-  "media.wmf.enabled",
-  "media.wmf.skip-blacklist",
-  "media.wmf.vp9.enabled",
-  "memory.free_dirty_pages",
-  "memory.low_commit_space_threshold_mb",
-  "memory.low_memory_notification_interval_ms",
-  "memory.low_physical_memory_threshold_mb",
-  "memory.low_virtual_mem_threshold_mb",
-  "network.IDN.blacklist_chars",
-  "network.IDN.restriction_profile",
-  "network.IDN.use_whitelist",
-  "network.IDN_show_punycode",
-  "network.buffer.cache.count",
-  "network.buffer.cache.size",
-  "network.captive-portal-service.enabled",
-  "network.cookie.cookieBehavior",
-  "network.cookie.lifetimePolicy",
-  "network.dns.disablePrefetch",
-  "network.dns.disablePrefetchFromHTTPS",
-  "network.jar.block-remote-files",
-  "network.loadinfo.skip_type_assertion",
-  "network.notify.changed",
-  "network.offline-mirrors-connectivity",
-  "network.protocol-handler.external.jar",
-  "network.proxy.type",
-  "network.security.ports.banned",
-  "network.security.ports.banned.override",
-  "network.standard-url.enable-rust",
-  "network.standard-url.max-length",
-  "network.sts.max_time_for_events_between_two_polls",
-  "network.sts.max_time_for_pr_close_during_shutdown",
-  "network.tcp.keepalive.enabled",
-  "network.tcp.keepalive.idle_time",
-  "network.tcp.keepalive.probe_count",
-  "network.tcp.keepalive.retry_interval",
-  "network.tcp.sendbuffer",
-  "nglayout.debug.invalidation",
-  "privacy.donottrackheader.enabled",
-  "privacy.firstparty.isolate",
-  "privacy.firstparty.isolate.restrict_opener_access",
-  "privacy.resistFingerprinting",
-  "security.data_uri.inherit_security_context",
-  "security.fileuri.strict_origin_policy",
-  "security.sandbox.content.level",
-  "security.sandbox.content.tempDirSuffix",
-  "security.sandbox.logging.enabled",
-  "security.sandbox.mac.track.violations",
-  "security.sandbox.windows.log",
-  "security.sandbox.windows.log.stackTraceDepth",
-  "shutdown.watchdog.timeoutSecs",
-  "signed.applets.codebase_principal_support",
-  "svg.disabled",
-  "svg.display-lists.hit-testing.enabled",
-  "svg.display-lists.painting.enabled",
-  "svg.new-getBBox.enabled",
-  "svg.paint-order.enabled",
-  "svg.path-caching.enabled",
-  "svg.transform-box.enabled",
-  "toolkit.asyncshutdown.crash_timeout",
-  "toolkit.asyncshutdown.log",
-  "toolkit.osfile.log",
-  "toolkit.osfile.log.redirect",
-  "toolkit.telemetry.enabled",
-  "toolkit.telemetry.idleTimeout",
-  "toolkit.telemetry.initDelay",
-  "toolkit.telemetry.log.dump",
-  "toolkit.telemetry.log.level",
-  "toolkit.telemetry.minSubsessionLength",
-  "toolkit.telemetry.scheduler.idleTickInterval",
-  "toolkit.telemetry.scheduler.tickInterval",
-  "toolkit.telemetry.unified",
-  "ui.key.menuAccessKeyFocuses",
-  "ui.popup.disable_autohide",
-  "ui.use_activity_cursor",
-  "view_source.editor.external"};
-
-const char** mozilla::dom::ContentPrefs::GetContentPrefs(size_t* aCount)
-{
-  *aCount = ArrayLength(ContentPrefs::gInitPrefs);
-  return gInitPrefs;
-}
-
-const char*  mozilla::dom::ContentPrefs::GetContentPref(size_t aIndex)
-{
-  MOZ_ASSERT(aIndex < ArrayLength(ContentPrefs::gInitPrefs));
-  return gInitPrefs[aIndex];
-}
-
diff --git a/dom/ipc/ContentPrefs.h b/dom/ipc/ContentPrefs.h
deleted file mode 100644
index f85d4b5cbad5..000000000000
--- a/dom/ipc/ContentPrefs.h
+++ /dev/null
@@ -1,25 +0,0 @@
-/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
-/* vim: set ts=8 sts=2 et sw=2 tw=80: */
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-#ifndef mozilla_dom_ContentPrefs_h
-#define mozilla_dom_ContentPrefs_h
-
-namespace mozilla {
-namespace dom {
-
-class ContentPrefs {
-public:
-  static const char** GetContentPrefs(size_t* aCount);
-  static const char* GetContentPref(size_t aIndex);
-
-private:
-    static const char* gInitPrefs[];
-};
-
-}
-}
-
-#endif
diff --git a/dom/ipc/ContentProcess.cpp b/dom/ipc/ContentProcess.cpp
index 394e7f1deff8..2413d88087e5 100644
--- a/dom/ipc/ContentProcess.cpp
+++ b/dom/ipc/ContentProcess.cpp
@@ -7,7 +7,6 @@
 #include "mozilla/ipc/IOThreadChild.h"
 
 #include "ContentProcess.h"
-#include "ContentPrefs.h"
 
 #if defined(XP_MACOSX) && defined(MOZ_CONTENT_SANDBOX)
 #include <stdlib.h>
@@ -104,143 +103,46 @@ SetUpSandboxEnvironment()
 }
 #endif
 
-bool
-ContentProcess::Init(int aArgc, char* aArgv[])
+void
+ContentProcess::SetAppDir(const nsACString& aPath)
 {
-  // If passed in grab the application path for xpcom init
-  bool foundAppdir = false;
-  bool foundChildID = false;
-  bool foundIsForBrowser = false;
-  bool foundIntPrefs = false;
-  bool foundBoolPrefs = false;
-  bool foundStringPrefs = false;
-
-  uint64_t childID;
-  bool isForBrowser;
+  mXREEmbed.SetAppDir(aPath);
+}
 
 #if defined(XP_MACOSX) && defined(MOZ_CONTENT_SANDBOX)
-  // If passed in grab the profile path for sandboxing
-  bool foundProfile = false;
-  nsCOMPtr<nsIFile> profileDir;
-#endif
-
-  InfallibleTArray<PrefSetting> prefsArray;
-  for (int idx = aArgc; idx > 0; idx--) {
-    if (!aArgv[idx]) {
-      continue;
-    }
-
-    if (!strcmp(aArgv[idx], "-appdir")) {
-      MOZ_ASSERT(!foundAppdir);
-      if (foundAppdir) {
-        continue;
-      }
-      nsCString appDir;
-      appDir.Assign(nsDependentCString(aArgv[idx+1]));
-      mXREEmbed.SetAppDir(appDir);
-      foundAppdir = true;
-    } else if (!strcmp(aArgv[idx], "-childID")) {
-      MOZ_ASSERT(!foundChildID);
-      if (foundChildID) {
-        continue;
-      }
-      if (idx + 1 < aArgc) {
-        childID = strtoull(aArgv[idx + 1], nullptr, 10);
-        foundChildID = true;
-      }
-    } else if (!strcmp(aArgv[idx], "-isForBrowser") || !strcmp(aArgv[idx], "-notForBrowser")) {
-      MOZ_ASSERT(!foundIsForBrowser);
-      if (foundIsForBrowser) {
-        continue;
-      }
-      isForBrowser = strcmp(aArgv[idx], "-notForBrowser");
-      foundIsForBrowser = true;
-    } else if (!strcmp(aArgv[idx], "-intPrefs")) {
-      SET_PREF_PHASE(BEGIN_INIT_PREFS);
-      char* str = aArgv[idx + 1];
-      while (*str) {
-        int32_t index = strtol(str, &str, 10);
-        str++;
-        MaybePrefValue value(PrefValue(static_cast<int32_t>(strtol(str, &str, 10))));
-        str++;
-        PrefSetting pref(nsCString(ContentPrefs::GetContentPref(index)), value, MaybePrefValue());
-        prefsArray.AppendElement(pref);
-      }
-      SET_PREF_PHASE(END_INIT_PREFS);
-      foundIntPrefs = true;
-    } else if (!strcmp(aArgv[idx], "-boolPrefs")) {
-      SET_PREF_PHASE(BEGIN_INIT_PREFS);
-      char* str = aArgv[idx + 1];
-      while (*str) {
-        int32_t index = strtol(str, &str, 10);
-        str++;
-        MaybePrefValue value(PrefValue(!!strtol(str, &str, 10)));
-        str++;
-        PrefSetting pref(nsCString(ContentPrefs::GetContentPref(index)), value, MaybePrefValue());
-        prefsArray.AppendElement(pref);
-      }
-      SET_PREF_PHASE(END_INIT_PREFS);
-      foundBoolPrefs = true;
-    } else if (!strcmp(aArgv[idx], "-stringPrefs")) {
-      SET_PREF_PHASE(BEGIN_INIT_PREFS);
-      char* str = aArgv[idx + 1];
-      while (*str) {
-        int32_t index = strtol(str, &str, 10);
-        str++;
-        int32_t length = strtol(str, &str, 10);
-        str++;
-        MaybePrefValue value(PrefValue(nsCString(str, length)));
-        PrefSetting pref(nsCString(ContentPrefs::GetContentPref(index)), value, MaybePrefValue());
-        prefsArray.AppendElement(pref);
-        str += length + 1;
-      }
-      SET_PREF_PHASE(END_INIT_PREFS);
-      foundStringPrefs = true;
-    }
-
-#if defined(XP_MACOSX) && defined(MOZ_CONTENT_SANDBOX)
-    else if (!strcmp(aArgv[idx], "-profile")) {
-      MOZ_ASSERT(!foundProfile);
-      if (foundProfile) {
-        continue;
-      }
-      bool flag;
-      nsresult rv = XRE_GetFileFromPath(aArgv[idx+1], getter_AddRefs(profileDir));
-      if (NS_FAILED(rv) ||
-          NS_FAILED(profileDir->Exists(&flag)) || !flag) {
-        NS_WARNING("Invalid profile directory passed to content process.");
-        profileDir = nullptr;
-      }
-      foundProfile = true;
-    }
-#endif /* XP_MACOSX && MOZ_CONTENT_SANDBOX */
-
-    bool allFound = foundAppdir && foundChildID && foundIsForBrowser && foundIntPrefs && foundBoolPrefs && foundStringPrefs;
-
-#if defined(XP_MACOSX) && defined(MOZ_CONTENT_SANDBOX)
-    allFound &= foundProfile;
-#endif
-
-    if (allFound) {
-      break;
-    }
+void
+ContentProcess::SetProfile(const nsACString& aProfile)
+{
+  bool flag;
+  nsresult rv =
+    XRE_GetFileFromPath(aProfile.BeginReading(), getter_AddRefs(mProfileDir));
+  if (NS_FAILED(rv) ||
+      NS_FAILED(mProfileDir->Exists(&flag)) || !flag) {
+    NS_WARNING("Invalid profile directory passed to content process.");
+    mProfileDir = nullptr;
   }
-  Preferences::SetInitPreferences(&prefsArray);
-  mContent.Init(IOThreadChild::message_loop(),
-                ParentPid(),
-                IOThreadChild::channel(),
-                childID,
-                isForBrowser);
-  mXREEmbed.Start();
+}
+#endif
+
+bool
+ContentProcess::Init()
+{
+    mContent.Init(IOThreadChild::message_loop(),
+                  ParentPid(),
+                  IOThreadChild::channel());
+    mXREEmbed.Start();
+    mContent.InitXPCOM();
+    mContent.InitGraphicsDeviceData();
+
 #if (defined(XP_MACOSX)) && defined(MOZ_CONTENT_SANDBOX)
-  mContent.SetProfileDir(profileDir);
+    mContent.SetProfileDir(mProfileDir);
 #endif
 
 #if (defined(XP_WIN) || defined(XP_MACOSX)) && defined(MOZ_CONTENT_SANDBOX)
-  SetUpSandboxEnvironment();
+    SetUpSandboxEnvironment();
 #endif
 
-  return true;
+    return true;
 }
 
 // Note: CleanUp() never gets called in non-debug builds because we exit early
@@ -248,7 +150,7 @@ ContentProcess::Init(int aArgc, char* aArgv[])
 void
 ContentProcess::CleanUp()
 {
-  mXREEmbed.Stop();
+    mXREEmbed.Stop();
 }
 
 } // namespace dom
diff --git a/dom/ipc/ContentProcess.h b/dom/ipc/ContentProcess.h
index a3854c761e10..bf9968f8cad9 100644
--- a/dom/ipc/ContentProcess.h
+++ b/dom/ipc/ContentProcess.h
@@ -34,13 +34,23 @@ public:
   ~ContentProcess()
   { }
 
-  virtual bool Init(int aArgc, char* aArgv[]) override;
+  virtual bool Init() override;
   virtual void CleanUp() override;
 
+  void SetAppDir(const nsACString& aPath);
+
+#if defined(XP_MACOSX) && defined(MOZ_CONTENT_SANDBOX)
+  void SetProfile(const nsACString& aProfile);
+#endif
+
 private:
   ContentChild mContent;
   mozilla::ipc::ScopedXREEmbed mXREEmbed;
 
+#if defined(XP_MACOSX) && defined(MOZ_CONTENT_SANDBOX)
+  nsCOMPtr<nsIFile> mProfileDir;
+#endif
+
 #if defined(XP_WIN)
   // This object initializes and configures COM.
   mozilla::mscom::MainThreadRuntime mCOMRuntime;
diff --git a/dom/ipc/PContent.ipdl b/dom/ipc/PContent.ipdl
index 71f382b4fb7b..eead95c2fa64 100644
--- a/dom/ipc/PContent.ipdl
+++ b/dom/ipc/PContent.ipdl
@@ -334,22 +334,6 @@ struct GMPCapabilityData
     GMPAPITags[] capabilities;
 };
 
-struct XPCOMInitData
-{
-    bool isOffline;
-    bool isConnected;
-    int32_t captivePortalState;
-    bool isLangRTL;
-    bool haveBidiKeyboards;
-    nsString[] dictionaries;
-    ClipboardCapabilities clipboardCaps;
-    DomainPolicyClone domainPolicy;
-    /* used on MacOSX only */
-    FontFamilyListEntry[] fontFamilies;
-    OptionalURIParams userContentSheetURL;
-    PrefSetting[] prefs;
-};
-
 struct GfxInfoFeatureStatus
 {
     int32_t feature;
@@ -543,8 +527,6 @@ child:
      */
     async InitBlobURLs(BlobURLRegistrationData[] registrations);
 
-    async SetXPCOMProcessAttributes(XPCOMInitData xpcomInit, StructuredCloneData initialData, LookAndFeelInt[] lookAndFeelIntCache);
-
     // Notify child that last-pb-context-exited notification was observed
     async LastPrivateDocShellDestroyed();
 
@@ -668,6 +650,30 @@ child:
 parent:
     async InitBackground(Endpoint<PBackgroundParent> aEndpoint);
 
+    /**
+     * Tell the content process some attributes of itself.  This is
+     * among the first information queried by content processes after
+     * startup.  (The message is sync to allow the content process to
+     * control when it receives the information.)
+     *
+     * |id| is a unique ID among all subprocesses. When
+     * |isForBrowser|, we're loading <browser> or <xul:browser remote>.
+     *
+     * Keep the return values in sync with PBrowser()!
+     */
+    sync GetProcessAttributes()
+        returns (ContentParentId cpId, bool isForBrowser);
+    sync GetXPCOMProcessAttributes()
+        returns (bool isOffline, bool isConnected, int32_t captivePortalState,
+                 bool isLangRTL,
+                 bool haveBidiKeyboards, nsString[] dictionaries,
+                 ClipboardCapabilities clipboardCaps,
+                 DomainPolicyClone domainPolicy,
+                 StructuredCloneData initialData,
+                 FontFamilyListEntry[] fontFamilies /* used on MacOSX only */,
+                 OptionalURIParams userContentSheetURL,
+                 LookAndFeelInt[] lookAndFeelIntCache);
+
     sync CreateChildProcess(IPCTabContext context,
                             ProcessPriority priority,
                             TabId openerTabId)
@@ -811,6 +817,7 @@ parent:
     async ExtProtocolChannelConnectParent(uint32_t registrarId);
 
     // PrefService message
+    sync ReadPrefsArray() returns (PrefSetting[] prefs) verify;
     sync GetGfxVars() returns (GfxVarUpdate[] vars);
 
     sync ReadFontList() returns (FontListEntry[] retValue);
diff --git a/dom/ipc/TabChild.cpp b/dom/ipc/TabChild.cpp
index a72655e9dd10..debd5cd2e25f 100644
--- a/dom/ipc/TabChild.cpp
+++ b/dom/ipc/TabChild.cpp
@@ -119,8 +119,6 @@
 #include "GroupedSHistory.h"
 #include "nsIHttpChannel.h"
 #include "mozilla/dom/DocGroup.h"
-#include "nsISupportsPrimitives.h"
-#include "mozilla/Telemetry.h"
 
 #ifdef NS_PRINTING
 #include "nsIPrintSession.h"
diff --git a/dom/ipc/moz.build b/dom/ipc/moz.build
index e84887ea40a8..452e1fe45166 100644
--- a/dom/ipc/moz.build
+++ b/dom/ipc/moz.build
@@ -20,7 +20,6 @@ EXPORTS.mozilla.dom += [
     'ContentBridgeParent.h',
     'ContentChild.h',
     'ContentParent.h',
-    'ContentPrefs.h',
     'ContentProcess.h',
     'ContentProcessManager.h',
     'CPOWManagerGetter.h',
@@ -51,7 +50,6 @@ UNIFIED_SOURCES += [
     'ContentBridgeChild.cpp',
     'ContentBridgeParent.cpp',
     'ContentParent.cpp',
-    'ContentPrefs.cpp',
     'ContentProcess.cpp',
     'ContentProcessManager.cpp',
     'CrashReporterParent.cpp',
diff --git a/dom/media/gmp/GMPProcessChild.cpp b/dom/media/gmp/GMPProcessChild.cpp
index d56339bc1018..d1ada706a7dd 100644
--- a/dom/media/gmp/GMPProcessChild.cpp
+++ b/dom/media/gmp/GMPProcessChild.cpp
@@ -25,7 +25,7 @@ GMPProcessChild::~GMPProcessChild()
 }
 
 bool
-GMPProcessChild::Init(int aArgc, char* aArgv[])
+GMPProcessChild::Init()
 {
   nsAutoString pluginFilename;
 
diff --git a/dom/media/gmp/GMPProcessChild.h b/dom/media/gmp/GMPProcessChild.h
index 382882e18a31..c2c8be9e2ebc 100644
--- a/dom/media/gmp/GMPProcessChild.h
+++ b/dom/media/gmp/GMPProcessChild.h
@@ -22,7 +22,7 @@ public:
   explicit GMPProcessChild(ProcessId aParentPid);
   ~GMPProcessChild();
 
-  bool Init(int aArgc, char* aArgv[]) override;
+  bool Init() override;
   void CleanUp() override;
 
 private:
diff --git a/dom/plugins/ipc/PluginProcessChild.cpp b/dom/plugins/ipc/PluginProcessChild.cpp
index 5b0a2beecdea..eb698e8af5cf 100644
--- a/dom/plugins/ipc/PluginProcessChild.cpp
+++ b/dom/plugins/ipc/PluginProcessChild.cpp
@@ -40,7 +40,7 @@ namespace plugins {
 
 
 bool
-PluginProcessChild::Init(int aArgc, char* aArgv[])
+PluginProcessChild::Init()
 {
     nsDebugImpl::SetMultiprocessMode("NPAPI");
 
diff --git a/dom/plugins/ipc/PluginProcessChild.h b/dom/plugins/ipc/PluginProcessChild.h
index fd0a328bc703..75c842a1bfda 100644
--- a/dom/plugins/ipc/PluginProcessChild.h
+++ b/dom/plugins/ipc/PluginProcessChild.h
@@ -30,7 +30,7 @@ public:
     virtual ~PluginProcessChild()
     { }
 
-    virtual bool Init(int aArgc, char* aArgv[]) override;
+    virtual bool Init() override;
     virtual void CleanUp() override;
 
 protected:
diff --git a/gfx/ipc/GPUProcessImpl.cpp b/gfx/ipc/GPUProcessImpl.cpp
index f7c4c80daafe..38d13484b3e7 100644
--- a/gfx/ipc/GPUProcessImpl.cpp
+++ b/gfx/ipc/GPUProcessImpl.cpp
@@ -22,7 +22,7 @@ GPUProcessImpl::~GPUProcessImpl()
 }
 
 bool
-GPUProcessImpl::Init(int aArgc, char* aArgv[])
+GPUProcessImpl::Init()
 {
   return mGPU.Init(ParentPid(),
                    IOThreadChild::message_loop(),
diff --git a/gfx/ipc/GPUProcessImpl.h b/gfx/ipc/GPUProcessImpl.h
index fc7ff8ca3a6e..5e8b6694a6c6 100644
--- a/gfx/ipc/GPUProcessImpl.h
+++ b/gfx/ipc/GPUProcessImpl.h
@@ -24,7 +24,7 @@ public:
   explicit GPUProcessImpl(ProcessId aParentPid);
   ~GPUProcessImpl();
 
-  bool Init(int aArgc, char* aArgv[]) override;
+  bool Init() override;
   void CleanUp() override;
 
 private:
diff --git a/ipc/glue/ProcessChild.h b/ipc/glue/ProcessChild.h
index 65d62bae8fba..4d1d3865902a 100644
--- a/ipc/glue/ProcessChild.h
+++ b/ipc/glue/ProcessChild.h
@@ -27,7 +27,7 @@ public:
   explicit ProcessChild(ProcessId aParentPid);
   virtual ~ProcessChild();
 
-  virtual bool Init(int aArgc, char* aArgv[]) = 0;
+  virtual bool Init() = 0;
   virtual void CleanUp()
   { }
 
diff --git a/modules/libpref/Preferences.cpp b/modules/libpref/Preferences.cpp
index b1f86b9e0f62..f36819236f47 100644
--- a/modules/libpref/Preferences.cpp
+++ b/modules/libpref/Preferences.cpp
@@ -5,7 +5,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/MemoryReporting.h"
-#include "mozilla/dom/PContent.h"
+#include "mozilla/dom/ContentChild.h"
 
 #include "mozilla/ArrayUtils.h"
 #include "mozilla/Attributes.h"
@@ -546,14 +546,6 @@ NS_INTERFACE_MAP_END
  * nsIPrefService Implementation
  */
 
-InfallibleTArray<Preferences::PrefSetting>* gInitPrefs;
-
-/*static*/
-void
-Preferences::SetInitPreferences(nsTArray<PrefSetting>* aPrefs) {
-  gInitPrefs = new InfallibleTArray<PrefSetting>(mozilla::Move(*aPrefs));
-}
-
 nsresult
 Preferences::Init()
 {
@@ -565,13 +557,15 @@ Preferences::Init()
   rv = pref_InitInitialObjects();
   NS_ENSURE_SUCCESS(rv, rv);
 
+  using mozilla::dom::ContentChild;
   if (XRE_IsContentProcess()) {
-    MOZ_ASSERT(gInitPrefs);
-    for (unsigned int i = 0; i < gInitPrefs->Length(); i++) {
-      Preferences::SetPreference(gInitPrefs->ElementAt(i));
+    InfallibleTArray<PrefSetting> prefs;
+    ContentChild::GetSingleton()->SendReadPrefsArray(&prefs);
+
+    // Store the array
+    for (uint32_t i = 0; i < prefs.Length(); ++i) {
+      pref_SetPref(prefs[i]);
     }
-    delete gInitPrefs;
-    gInitPrefs = nullptr;
     return NS_OK;
   }
 
@@ -786,14 +780,6 @@ Preferences::GetPreferences(InfallibleTArray<PrefSetting>* aPrefs)
   }
 }
 
-#ifdef DEBUG
-void
-Preferences::SetInitPhase(pref_initPhase phase)
-{
-  pref_SetInitPhase(phase);
-}
-#endif
-
 NS_IMETHODIMP
 Preferences::GetBranch(const char *aPrefRoot, nsIPrefBranch **_retval)
 {
diff --git a/modules/libpref/Preferences.h b/modules/libpref/Preferences.h
index 8b0daef13b46..255d2a8d2f29 100644
--- a/modules/libpref/Preferences.h
+++ b/modules/libpref/Preferences.h
@@ -28,20 +28,6 @@ typedef void (*PrefChangedFunc)(const char *, void *);
 #define have_PrefChangedFunc_typedef
 #endif
 
-#ifdef DEBUG
-enum pref_initPhase {
-  START,
-  BEGIN_INIT_PREFS,
-  END_INIT_PREFS,
-  BEGIN_ALL_PREFS,
-  END_ALL_PREFS
-};
-
-#define SET_PREF_PHASE(p) Preferences::SetInitPhase(p)
-#else
-#define SET_PREF_PHASE(p) do { } while (0)
-#endif
-
 namespace mozilla {
 
 namespace dom {
@@ -380,12 +366,6 @@ public:
   static void GetPreference(PrefSetting* aPref);
   static void SetPreference(const PrefSetting& aPref);
 
-  static void SetInitPreferences(nsTArray<PrefSetting>* aPrefs);
-
-#ifdef DEBUG
-  static void SetInitPhase(pref_initPhase phase);
-#endif
-
   static int64_t SizeOfIncludingThisAndOtherStuff(mozilla::MallocSizeOf aMallocSizeOf);
 
   static void DirtyCallback();
diff --git a/modules/libpref/prefapi.cpp b/modules/libpref/prefapi.cpp
index a42c83d343bf..046a6941903d 100644
--- a/modules/libpref/prefapi.cpp
+++ b/modules/libpref/prefapi.cpp
@@ -29,7 +29,6 @@
 #include "prprf.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/dom/PContent.h"
-#include "mozilla/dom/ContentPrefs.h"
 #include "nsQuickSort.h"
 #include "nsString.h"
 #include "nsPrintfCString.h"
@@ -737,48 +736,13 @@ static PrefTypeFlags pref_SetValue(PrefValue* existingValue, PrefTypeFlags flags
     }
     return flags;
 }
-#ifdef DEBUG
-static pref_initPhase gPhase = START;
-
-void
-pref_SetInitPhase(pref_initPhase phase)
-{
-    gPhase = phase;
-}
-
-struct StringComparator
-{
-    const char* mKey;
-    explicit StringComparator(const char* aKey) : mKey(aKey) {}
-    int operator()(const char* string) const {
-        return strcmp(mKey, string);
-    }
-};
-
-bool
-inInitArray(const char* key)
-{
-    size_t prefsLen;
-    size_t found;
-    const char** list = mozilla::dom::ContentPrefs::GetContentPrefs(&prefsLen);
-    return BinarySearchIf(list, 0, prefsLen,
-                          StringComparator(key), &found);
-}
-#endif
 
 PrefHashEntry* pref_HashTableLookup(const char *key)
 {
 #ifndef MOZ_B2G
     MOZ_ASSERT(NS_IsMainThread());
 #endif
-    MOZ_ASSERT((!XRE_IsContentProcess() || gPhase != START),
-               "pref access before commandline prefs set");
-    /* If you're hitting this assertion, you've added a pref access to start up.
-     * Consider moving it later or add it to the whitelist in ContentPrefs.cpp
-     * and get review from a DOM peer
-     */
-    MOZ_ASSERT((!XRE_IsContentProcess() || gPhase > END_INIT_PREFS || inInitArray(key)),
-               "accessing non-init pref before the rest of the prefs are sent");
+
     return static_cast<PrefHashEntry*>(gHashTable->Search(key));
 }
 
diff --git a/modules/libpref/prefapi_private_data.h b/modules/libpref/prefapi_private_data.h
index 4cf58f6a9f85..f1fa68fdc45e 100644
--- a/modules/libpref/prefapi_private_data.h
+++ b/modules/libpref/prefapi_private_data.h
@@ -10,7 +10,6 @@
 
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/UniquePtr.h"
-#include "Preferences.h"
 
 extern PLDHashTable* gHashTable;
 
@@ -26,11 +25,6 @@ pref_savePrefs(PLDHashTable* aTable, uint32_t* aPrefCount);
 nsresult
 pref_SetPref(const mozilla::dom::PrefSetting& aPref);
 
-#ifdef DEBUG
-void
-pref_SetInitPhase(pref_initPhase phase);
-#endif
-
 int pref_CompareStrings(const void *v1, const void *v2, void* unused);
 PrefHashEntry* pref_HashTableLookup(const char *key);
 
diff --git a/toolkit/xre/nsEmbedFunctions.cpp b/toolkit/xre/nsEmbedFunctions.cpp
index 4649a41525f8..89584b5621ad 100644
--- a/toolkit/xre/nsEmbedFunctions.cpp
+++ b/toolkit/xre/nsEmbedFunctions.cpp
@@ -625,8 +625,49 @@ XRE_InitChildProcess(int aArgc,
         process = new PluginProcessChild(parentPID);
         break;
 
-      case GeckoProcessType_Content:
-        process = new ContentProcess(parentPID);
+      case GeckoProcessType_Content: {
+          process = new ContentProcess(parentPID);
+          // If passed in grab the application path for xpcom init
+          bool foundAppdir = false;
+
+#if defined(XP_MACOSX) && defined(MOZ_CONTENT_SANDBOX)
+          // If passed in grab the profile path for sandboxing
+          bool foundProfile = false;
+#endif
+
+          for (int idx = aArgc; idx > 0; idx--) {
+            if (aArgv[idx] && !strcmp(aArgv[idx], "-appdir")) {
+              MOZ_ASSERT(!foundAppdir);
+              if (foundAppdir) {
+                  continue;
+              }
+              nsCString appDir;
+              appDir.Assign(nsDependentCString(aArgv[idx+1]));
+              static_cast<ContentProcess*>(process.get())->SetAppDir(appDir);
+              foundAppdir = true;
+            }
+
+#if defined(XP_MACOSX) && defined(MOZ_CONTENT_SANDBOX)
+            if (aArgv[idx] && !strcmp(aArgv[idx], "-profile")) {
+              MOZ_ASSERT(!foundProfile);
+              if (foundProfile) {
+                continue;
+              }
+              nsCString profile;
+              profile.Assign(nsDependentCString(aArgv[idx+1]));
+              static_cast<ContentProcess*>(process.get())->SetProfile(profile);
+              foundProfile = true;
+            }
+            if (foundProfile && foundAppdir) {
+              break;
+            }
+#else
+            if (foundAppdir) {
+              break;
+            }
+#endif /* XP_MACOSX && MOZ_CONTENT_SANDBOX */
+          }
+        }
         break;
 
       case GeckoProcessType_IPDLUnitTest:
@@ -649,7 +690,7 @@ XRE_InitChildProcess(int aArgc,
         MOZ_CRASH("Unknown main thread class");
       }
 
-      if (!process->Init(aArgc, aArgv)) {
+      if (!process->Init()) {
         return NS_ERROR_FAILURE;
       }