diff --git a/dom/ipc/ContentParent.cpp b/dom/ipc/ContentParent.cpp
index 7cc676bb53a3..cdf579754728 100644
--- a/dom/ipc/ContentParent.cpp
+++ b/dom/ipc/ContentParent.cpp
@@ -2056,6 +2056,7 @@ void ContentParent::LaunchSubprocessInternal(
     earlyReject();
     return;
   }
+  prefSerializer.AddSharedPrefCmdLineArgs(*mSubprocess, extraArgs);
 
   // Register ContentParent as an observer for changes to any pref
   // whose prefix matches the empty string, i.e. all of them.  The
@@ -2063,43 +2064,6 @@ void ContentParent::LaunchSubprocessInternal(
   // happen during async launch.
   Preferences::AddStrongObserver(this, "");
 
-  // Formats a pointer or pointer-sized-integer as a string suitable for passing
-  // in an arguments list.
-  auto formatPtrArg = [](auto arg) {
-    return nsPrintfCString("%zu", uintptr_t(arg));
-  };
-
-#if defined(XP_WIN)
-  // Record the handle as to-be-shared, and pass it via a command flag. This
-  // works because Windows handles are system-wide.
-  HANDLE prefsHandle = prefSerializer.GetSharedMemoryHandle();
-  mSubprocess->AddHandleToShare(prefsHandle);
-  mSubprocess->AddHandleToShare(prefSerializer.GetPrefMapHandle().get());
-  extraArgs.push_back("-prefsHandle");
-  extraArgs.push_back(formatPtrArg(prefsHandle).get());
-  extraArgs.push_back("-prefMapHandle");
-  extraArgs.push_back(
-      formatPtrArg(prefSerializer.GetPrefMapHandle().get()).get());
-#else
-  // In contrast, Unix fds are per-process. So remap the fd to a fixed one that
-  // will be used in the child.
-  // XXX: bug 1440207 is about improving how fixed fds are used.
-  //
-  // Note: on Android, AddFdToRemap() sets up the fd to be passed via a Parcel,
-  // and the fixed fd isn't used. However, we still need to mark it for
-  // remapping so it doesn't get closed in the child.
-  mSubprocess->AddFdToRemap(prefSerializer.GetSharedMemoryHandle().fd,
-                            kPrefsFileDescriptor);
-  mSubprocess->AddFdToRemap(prefSerializer.GetPrefMapHandle().get(),
-                            kPrefMapFileDescriptor);
-#endif
-
-  // Pass the lengths via command line flags.
-  extraArgs.push_back("-prefsLen");
-  extraArgs.push_back(formatPtrArg(prefSerializer.GetPrefLength()).get());
-  extraArgs.push_back("-prefMapSize");
-  extraArgs.push_back(formatPtrArg(prefSerializer.GetPrefMapSize()).get());
-
   if (gSafeMode) {
     extraArgs.push_back("-safeMode");
   }
diff --git a/ipc/glue/ProcessUtils.h b/ipc/glue/ProcessUtils.h
index c05358223779..8119f9ac40dc 100644
--- a/ipc/glue/ProcessUtils.h
+++ b/ipc/glue/ProcessUtils.h
@@ -13,6 +13,8 @@
 namespace mozilla {
 namespace ipc {
 
+class GeckoChildProcessHost;
+
 // You probably should call ContentChild::SetProcessName instead of calling
 // this directly.
 void SetThisProcessName(const char* aName);
@@ -37,6 +39,9 @@ class SharedPreferenceSerializer final {
 
   size_t GetPrefMapSize() const { return mPrefMapSize; }
 
+  void AddSharedPrefCmdLineArgs(GeckoChildProcessHost& procHost,
+                                std::vector<std::string>& aExtraOpts) const;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(SharedPreferenceSerializer);
   size_t mPrefMapSize;
diff --git a/ipc/glue/ProcessUtils_common.cpp b/ipc/glue/ProcessUtils_common.cpp
index e2f3de396d3b..bba91288194c 100644
--- a/ipc/glue/ProcessUtils_common.cpp
+++ b/ipc/glue/ProcessUtils_common.cpp
@@ -7,6 +7,7 @@
 #include "ProcessUtils.h"
 
 #include "mozilla/Preferences.h"
+#include "mozilla/ipc/GeckoChildProcessHost.h"
 
 namespace mozilla {
 namespace ipc {
@@ -51,6 +52,46 @@ bool SharedPreferenceSerializer::SerializeToSharedMemory() {
   return true;
 }
 
+void SharedPreferenceSerializer::AddSharedPrefCmdLineArgs(
+    mozilla::ipc::GeckoChildProcessHost& procHost,
+    std::vector<std::string>& aExtraOpts) const {
+  // Formats a pointer or pointer-sized-integer as a string suitable for passing
+  // in an arguments list.
+  auto formatPtrArg = [](auto arg) {
+    return nsPrintfCString("%zu", uintptr_t(arg));
+  };
+
+#if defined(XP_WIN)
+  // Record the handle as to-be-shared, and pass it via a command flag. This
+  // works because Windows handles are system-wide.
+  HANDLE prefsHandle = GetSharedMemoryHandle();
+  procHost.AddHandleToShare(prefsHandle);
+  procHost.AddHandleToShare(GetPrefMapHandle().get());
+  aExtraOpts.push_back("-prefsHandle");
+  aExtraOpts.push_back(formatPtrArg(prefsHandle).get());
+  aExtraOpts.push_back("-prefMapHandle");
+  aExtraOpts.push_back(
+      formatPtrArg(GetPrefMapHandle().get()).get());
+#else
+  // In contrast, Unix fds are per-process. So remap the fd to a fixed one that
+  // will be used in the child.
+  // XXX: bug 1440207 is about improving how fixed fds are used.
+  //
+  // Note: on Android, AddFdToRemap() sets up the fd to be passed via a Parcel,
+  // and the fixed fd isn't used. However, we still need to mark it for
+  // remapping so it doesn't get closed in the child.
+  procHost.AddFdToRemap(GetSharedMemoryHandle().fd, kPrefsFileDescriptor);
+  procHost.AddFdToRemap(GetPrefMapHandle().get(), kPrefMapFileDescriptor);
+#endif
+
+  // Pass the lengths via command line flags.
+  aExtraOpts.push_back("-prefsLen");
+  aExtraOpts.push_back(formatPtrArg(GetPrefLength()).get());
+  aExtraOpts.push_back("-prefMapSize");
+  aExtraOpts.push_back(formatPtrArg(GetPrefMapSize()).get());
+}
+
+
 #ifdef ANDROID
 static int gPrefsFd = -1;
 static int gPrefMapFd = -1;
diff --git a/netwerk/ipc/SocketProcessHost.cpp b/netwerk/ipc/SocketProcessHost.cpp
index 58fa85bc7149..448425c0fd2a 100644
--- a/netwerk/ipc/SocketProcessHost.cpp
+++ b/netwerk/ipc/SocketProcessHost.cpp
@@ -111,41 +111,7 @@ bool SocketProcessHost::Launch() {
   if (!prefSerializer.SerializeToSharedMemory()) {
     return false;
   }
-
-  // Formats a pointer or pointer-sized-integer as a string suitable for passing
-  // in an arguments list.
-  auto formatPtrArg = [](auto arg) {
-    return nsPrintfCString("%zu", uintptr_t(arg));
-  };
-
-#if defined(XP_WIN)
-  // Record the handle as to-be-shared, and pass it via a command flag. This
-  // works because Windows handles are system-wide.
-  HANDLE prefsHandle = prefSerializer.GetSharedMemoryHandle();
-  AddHandleToShare(prefsHandle);
-  AddHandleToShare(prefSerializer.GetPrefMapHandle().get());
-  extraArgs.push_back("-prefsHandle");
-  extraArgs.push_back(formatPtrArg(prefsHandle).get());
-  extraArgs.push_back("-prefMapHandle");
-  extraArgs.push_back(
-      formatPtrArg(prefSerializer.GetPrefMapHandle().get()).get());
-#else
-  // In contrast, Unix fds are per-process. So remap the fd to a fixed one that
-  // will be used in the child.
-  // XXX: bug 1440207 is about improving how fixed fds are used.
-  //
-  // Note: on Android, AddFdToRemap() sets up the fd to be passed via a Parcel,
-  // and the fixed fd isn't used. However, we still need to mark it for
-  // remapping so it doesn't get closed in the child.
-  AddFdToRemap(prefSerializer.GetSharedMemoryHandle().fd, kPrefsFileDescriptor);
-  AddFdToRemap(prefSerializer.GetPrefMapHandle().get(), kPrefMapFileDescriptor);
-#endif
-
-  // Pass the lengths via command line flags.
-  extraArgs.push_back("-prefsLen");
-  extraArgs.push_back(formatPtrArg(prefSerializer.GetPrefLength()).get());
-  extraArgs.push_back("-prefMapSize");
-  extraArgs.push_back(formatPtrArg(prefSerializer.GetPrefMapSize()).get());
+  prefSerializer.AddSharedPrefCmdLineArgs(*this, extraArgs);
 
   mLaunchPhase = LaunchPhase::Waiting;
   if (!GeckoChildProcessHost::LaunchAndWaitForProcessHandle(extraArgs)) {