--- a/dom/ipc/ContentParent.cpp
+++ b/dom/ipc/ContentParent.cpp
@@ -2325,17 +2325,20 @@ ContentParent::LaunchSubprocess(ProcessP
   extraArgs.push_back("-schedulerPrefs");
   extraArgs.push_back(schedulerPrefs.get());
 
   if (gSafeMode) {
     extraArgs.push_back("-safeMode");
   }
 
 #if defined(XP_MACOSX) && defined(MOZ_CONTENT_SANDBOX)
-  if (sEarlySandboxInit && IsContentSandboxEnabled()) {
+  // If we're launching a middleman process for a
+  // recording or replay, start the sandbox later.
+  if (sEarlySandboxInit && IsContentSandboxEnabled() &&
+      !IsRecordingOrReplaying()) {
     AppendSandboxParams(extraArgs);
   }
 #endif
 
   nsCString parentBuildID(mozilla::PlatformBuildID());
   extraArgs.push_back("-parentBuildID");
   extraArgs.push_back(parentBuildID.get());
 
@@ -2737,17 +2740,20 @@ ContentParent::InitInternal(ProcessPrior
   // purpose. If the decision is made to permanently rely on the pref, this
   // should be changed so that it is required to restart firefox for the change
   // of value to take effect.
   shouldSandbox = IsContentSandboxEnabled();
 
 #ifdef XP_MACOSX
   // If the sandbox was initialized during content process
   // startup, we must not send the SetProcessSandbox message.
-  shouldSandbox = shouldSandbox && !sEarlySandboxInit;
+  // If early startup was pref'd off or the process is a
+  // middleman process, send SetProcessSandbox now.
+  shouldSandbox = shouldSandbox &&
+    (!sEarlySandboxInit || IsRecordingOrReplaying());
 #endif
 
 #ifdef XP_LINUX
   if (shouldSandbox) {
     MOZ_ASSERT(!mSandboxBroker);
     bool isFileProcess = mRemoteType.EqualsLiteral(FILE_REMOTE_TYPE);
     UniquePtr<SandboxBroker::Policy> policy =
       sSandboxBrokerPolicyFactory->GetContentPolicy(Pid(), isFileProcess);

--- a/dom/ipc/ContentProcess.cpp
+++ b/dom/ipc/ContentProcess.cpp
@@ -293,18 +293,21 @@ ContentProcess::Init(int aArgc, char* aA
                 IOThreadChild::channel(),
                 *childID,
                 *isForBrowser);
 
   mXREEmbed.Start();
 #if (defined(XP_MACOSX)) && defined(MOZ_CONTENT_SANDBOX)
   mContent.SetProfileDir(profileDir);
 #if defined(DEBUG)
+  // For WebReplay middleman processes, the sandbox is
+  // started after receiving the SetProcessSandbox message.
   if (IsContentSandboxEnabled() &&
-      Preferences::GetBool("security.sandbox.content.mac.earlyinit")) {
+      Preferences::GetBool("security.sandbox.content.mac.earlyinit") &&
+      !recordreplay::IsMiddleman()) {
     AssertMacSandboxEnabled();
   }
 #endif /* DEBUG */
 #endif /* XP_MACOSX && MOZ_CONTENT_SANDBOX */
 
 #if defined(XP_WIN) && defined(MOZ_CONTENT_SANDBOX)
   SetUpSandboxEnvironment();
 #endif