diff --git a/dom/ipc/ContentProcess.cpp b/dom/ipc/ContentProcess.cpp
index 74a745e1a997..e3b89a4eb68a 100644
--- a/dom/ipc/ContentProcess.cpp
+++ b/dom/ipc/ContentProcess.cpp
@@ -14,6 +14,7 @@
 
 #if defined(XP_MACOSX) && defined(MOZ_CONTENT_SANDBOX)
 #include <stdlib.h>
+#include "mozilla/Sandbox.h"
 #endif
 
 #if (defined(XP_WIN) || defined(XP_MACOSX)) && defined(MOZ_CONTENT_SANDBOX)
@@ -296,7 +297,13 @@ ContentProcess::Init(int aArgc, char* aArgv[])
   mXREEmbed.Start();
 #if (defined(XP_MACOSX)) && defined(MOZ_CONTENT_SANDBOX)
   mContent.SetProfileDir(profileDir);
-#endif
+#if defined(DEBUG)
+  if (IsContentSandboxEnabled() &&
+      Preferences::GetBool("security.sandbox.content.mac.earlyinit")) {
+    AssertMacSandboxEnabled();
+  }
+#endif /* DEBUG */
+#endif /* XP_MACOSX && MOZ_CONTENT_SANDBOX */
 
 #if defined(XP_WIN) && defined(MOZ_CONTENT_SANDBOX)
   SetUpSandboxEnvironment();
diff --git a/security/sandbox/mac/Sandbox.h b/security/sandbox/mac/Sandbox.h
index 2ea6784e6cc4..efa3e61cd283 100644
--- a/security/sandbox/mac/Sandbox.h
+++ b/security/sandbox/mac/Sandbox.h
@@ -78,6 +78,9 @@ namespace mozilla {
 bool StartMacSandbox(MacSandboxInfo const &aInfo, std::string &aErrorMessage);
 bool EarlyStartMacSandboxIfEnabled(int aArgc, char** aArgv,
                                    std::string &aErrorMessage);
+#ifdef DEBUG
+void AssertMacSandboxEnabled();
+#endif /* DEBUG */
 
 } // namespace mozilla
 
diff --git a/security/sandbox/mac/Sandbox.mm b/security/sandbox/mac/Sandbox.mm
index d57387783937..2ef3c29502ae 100644
--- a/security/sandbox/mac/Sandbox.mm
+++ b/security/sandbox/mac/Sandbox.mm
@@ -489,5 +489,20 @@ EarlyStartMacSandboxIfEnabled(int aArgc, char** aArgv,
   return StartMacSandbox(info, aErrorMessage);
 }
 
+#ifdef DEBUG
+/*
+ * Ensures that a process sandbox is enabled by attempting to enable
+ * a new sandbox policy and ASSERT'ing that this fails. This depends
+ * on sandbox_init() failing when called again after a sandbox has
+ * already been successfully enabled.
+ */
+void
+AssertMacSandboxEnabled()
+{
+  char *errorbuf = NULL;
+  int rv = sandbox_init("(version 1)(deny default)", 0, &errorbuf);
+  MOZ_ASSERT(rv != 0);
+}
+#endif /* DEBUG */
 
 } // namespace mozilla