diff --git a/dom/ipc/ContentParent.cpp b/dom/ipc/ContentParent.cpp
index 81c0471ede49..6b6fba7edc79 100644
--- a/dom/ipc/ContentParent.cpp
+++ b/dom/ipc/ContentParent.cpp
@@ -79,6 +79,7 @@
 #include "mozilla/Components.h"
 #include "mozilla/Sprintf.h"
 #include "mozilla/StaticPrefs_dom.h"
+#include "mozilla/StaticPrefs_fission.h"
 #include "mozilla/StaticPrefs_media.h"
 #include "mozilla/StaticPrefs_widget.h"
 #include "mozilla/StyleSheet.h"
@@ -647,6 +648,23 @@ static const char* sObserverTopics[] = {
     "network:socket-process-crashed",
 };
 
+static const char kFissionEnforceBlockList[] =
+    "fission.enforceBlocklistedPrefsInSubprocesses";
+static const char kFissionOmitBlockListValues[] =
+    "fission.omitBlocklistedPrefsInSubprocesses";
+
+static void OnFissionBlocklistPrefChange(const char* aPref, void* aData) {
+  if (strcmp(aPref, kFissionEnforceBlockList) == 0) {
+    sCrashOnBlocklistedPref =
+        StaticPrefs::fission_enforceBlocklistedPrefsInSubprocesses();
+  } else if (strcmp(aPref, kFissionOmitBlockListValues) == 0) {
+    sOmitBlocklistedPrefValues =
+        StaticPrefs::fission_omitBlocklistedPrefsInSubprocesses();
+  } else {
+    MOZ_CRASH("Unknown pref passed to callback");
+  }
+}
+
 // PreallocateProcess is called by the PreallocatedProcessManager.
 // ContentParent then takes this process back within GetNewOrUsedBrowserProcess.
 /*static*/ already_AddRefed<ContentParent>
@@ -676,6 +694,11 @@ void ContentParent::StartUp() {
   BackgroundChild::Startup();
   ClientManager::Startup();
 
+  Preferences::RegisterCallbackAndCall(&OnFissionBlocklistPrefChange,
+                                       kFissionEnforceBlockList);
+  Preferences::RegisterCallbackAndCall(&OnFissionBlocklistPrefChange,
+                                       kFissionOmitBlockListValues);
+
 #if defined(XP_LINUX) && defined(MOZ_SANDBOX)
   sSandboxBrokerPolicyFactory = MakeUnique<SandboxBrokerPolicyFactory>();
 #endif
diff --git a/modules/libpref/Preferences.cpp b/modules/libpref/Preferences.cpp
index 57498a556fe2..b713d5a0c199 100644
--- a/modules/libpref/Preferences.cpp
+++ b/modules/libpref/Preferences.cpp
@@ -517,7 +517,7 @@ class Pref {
     MOZ_ASSERT(aKind == PrefValueKind::Default ? HasDefaultValue()
                                                : HasUserValue());
 
-    if (!XRE_IsParentProcess() &&
+    if (!XRE_IsParentProcess() && sCrashOnBlocklistedPref &&
         ShouldSanitizePreference(Name(), XRE_IsContentProcess())) {
       MOZ_CRASH_UNSAFE_PRINTF(
           "Should not access the preference '%s' in the Content Processes",
@@ -533,7 +533,7 @@ class Pref {
     MOZ_ASSERT(aKind == PrefValueKind::Default ? HasDefaultValue()
                                                : HasUserValue());
 
-    if (!XRE_IsParentProcess() &&
+    if (!XRE_IsParentProcess() && sCrashOnBlocklistedPref &&
         ShouldSanitizePreference(Name(), XRE_IsContentProcess())) {
       MOZ_CRASH_UNSAFE_PRINTF(
           "Should not access the preference '%s' in the Content Processes",
@@ -550,7 +550,7 @@ class Pref {
     MOZ_ASSERT(aKind == PrefValueKind::Default ? HasDefaultValue()
                                                : HasUserValue());
 
-    if (!XRE_IsParentProcess() &&
+    if (!XRE_IsParentProcess() && sCrashOnBlocklistedPref &&
         ShouldSanitizePreference(Name(), XRE_IsContentProcess())) {
       MOZ_CRASH_UNSAFE_PRINTF(
           "Should not access the preference '%s' in the Content Processes",
@@ -583,7 +583,8 @@ class Pref {
       aDomPref->defaultValue() = Nothing();
     }
 
-    if (mHasUserValue && !aDomPref->isSanitized()) {
+    if (mHasUserValue &&
+        !(aDomPref->isSanitized() && sOmitBlocklistedPrefValues)) {
       aDomPref->userValue() = Some(dom::PrefValue());
       mUserValue.ToDomPrefValue(Type(), &aDomPref->userValue().ref());
     } else {
@@ -591,7 +592,8 @@ class Pref {
     }
 
     MOZ_ASSERT(aDomPref->defaultValue().isNothing() ||
-               aDomPref->userValue().isNothing() || mIsSanitized ||
+               aDomPref->userValue().isNothing() ||
+               (mIsSanitized && sOmitBlocklistedPrefValues) ||
                (aDomPref->defaultValue().ref().type() ==
                 aDomPref->userValue().ref().type()));
   }
@@ -829,7 +831,7 @@ class Pref {
     }
     aStr.Append(':');
 
-    if (mHasUserValue && !aSanitizeUserValue) {
+    if (mHasUserValue && !(aSanitizeUserValue && sOmitBlocklistedPrefValues)) {
       mUserValue.SerializeAndAppend(Type(), aStr);
     }
     aStr.Append('\n');
@@ -5766,6 +5768,9 @@ bool ShouldSanitizePreference(const char* aPref,
   return Preferences::IsSanitized(aPref);
 }
 
+Atomic<bool, Relaxed> sOmitBlocklistedPrefValues(false);
+Atomic<bool, Relaxed> sCrashOnBlocklistedPref(false);
+
 }  // namespace mozilla
 
 // This file contains the C wrappers for the C++ static pref getters, as used
diff --git a/modules/libpref/Preferences.h b/modules/libpref/Preferences.h
index 96d85d6fa5a2..58b7430ed29e 100644
--- a/modules/libpref/Preferences.h
+++ b/modules/libpref/Preferences.h
@@ -536,6 +536,9 @@ class Preferences final : public nsIPrefService,
   static bool InitStaticMembers();
 };
 
+extern Atomic<bool, Relaxed> sOmitBlocklistedPrefValues;
+extern Atomic<bool, Relaxed> sCrashOnBlocklistedPref;
+
 bool ShouldSanitizePreference(const char* aPref, bool aIsDestWebContentProcess);
 
 }  // namespace mozilla
diff --git a/modules/libpref/init/StaticPrefList.yaml b/modules/libpref/init/StaticPrefList.yaml
index 7b0ddf8196ea..763a349356c8 100644
--- a/modules/libpref/init/StaticPrefList.yaml
+++ b/modules/libpref/init/StaticPrefList.yaml
@@ -4697,6 +4697,18 @@
   value: @IS_ANDROID@
   mirror: always
 
+# If true, do not send blocklisted preference values to the subprocess
+- name: fission.omitBlocklistedPrefsInSubprocesses
+  type: RelaxedAtomicBool
+  value: false
+  mirror: always
+
+# If true, crash when a blocklisted preference is accessed in a subprocess
+- name: fission.enforceBlocklistedPrefsInSubprocesses
+  type: RelaxedAtomicBool
+  value: false
+  mirror: always
+
 #---------------------------------------------------------------------------
 # Prefs starting with "font."
 #---------------------------------------------------------------------------
diff --git a/modules/libpref/init/StaticPrefListBegin.h b/modules/libpref/init/StaticPrefListBegin.h
index 2f37185b8ad1..8af91795549b 100644
--- a/modules/libpref/init/StaticPrefListBegin.h
+++ b/modules/libpref/init/StaticPrefListBegin.h
@@ -22,7 +22,8 @@ namespace StaticPrefs {
 #define ALWAYS_PREF(name, base_id, full_id, cpp_type, default_value)           \
   extern cpp_type sMirror_##full_id;                                           \
   inline StripAtomic<cpp_type> full_id() {                                     \
-    if (!XRE_IsParentProcess() && IsString<cpp_type>::value) {                 \
+    if (!XRE_IsParentProcess() && IsString<cpp_type>::value &&                 \
+        sCrashOnBlocklistedPref) {                                             \
       MOZ_DIAGNOSTIC_ASSERT(                                                   \
           !ShouldSanitizePreference(name, XRE_IsContentProcess()),             \
           "Should not access the preference '" name "' in Content Processes"); \
@@ -40,7 +41,8 @@ namespace StaticPrefs {
   extern cpp_type sMirror_##full_id;                                           \
   inline cpp_type full_id() {                                                  \
     MaybeInitOncePrefs();                                                      \
-    if (!XRE_IsParentProcess() && IsString<cpp_type>::value) {                 \
+    if (!XRE_IsParentProcess() && IsString<cpp_type>::value &&                 \
+        sCrashOnBlocklistedPref) {                                             \
       MOZ_DIAGNOSTIC_ASSERT(                                                   \
           !ShouldSanitizePreference(name, XRE_IsContentProcess()),             \
           "Should not access the preference '" name "' in Content Processes"); \