--- a/toolkit/xre/nsAppRunner.cpp
+++ b/toolkit/xre/nsAppRunner.cpp
@@ -4635,23 +4635,25 @@ enum {
   kE10sEnabledByUser = 0,
   kE10sEnabledByDefault = 1,
   kE10sDisabledByUser = 2,
   // kE10sDisabledInSafeMode = 3, was removed in bug 1172491.
   kE10sDisabledForAccessibility = 4,
   kE10sDisabledForMacGfx = 5,
   kE10sDisabledForBidi = 6,
   kE10sDisabledForAddons = 7,
+  kE10sForceDisabled = 8,
 };
 
 #ifdef XP_WIN
 const char* kAccessibilityLastRunDatePref = "accessibility.lastLoadDate";
 const char* kAccessibilityLoadedLastSessionPref = "accessibility.loadedInLastSession";
 #endif // XP_WIN
 const char* kForceEnableE10sPref = "browser.tabs.remote.force-enable";
+const char* kForceDisableE10sPref = "browser.tabs.remote.force-disable";
 
 #ifdef XP_WIN
 static inline uint32_t
 PRTimeToSeconds(PRTime t_usec)
 {
   PRTime usec_per_sec = PR_USEC_PER_SEC;
   return uint32_t(t_usec /= usec_per_sec);
 }
@@ -4802,16 +4804,23 @@ mozilla::BrowserTabsRemoteAutostart()
 
   // Uber override pref for manual testing purposes
   if (Preferences::GetBool(kForceEnableE10sPref, false)) {
     gBrowserTabsRemoteAutostart = true;
     prefEnabled = true;
     status = kE10sEnabledByUser;
   }
 
+  // Uber override pref for emergency blocking
+  if (gBrowserTabsRemoteAutostart &&
+      Preferences::GetBool(kForceDisableE10sPref, false)) {
+    gBrowserTabsRemoteAutostart = false;
+    status = kE10sForceDisabled;
+  }
+
   gBrowserTabsRemoteStatus = status;
 
   mozilla::Telemetry::Accumulate(mozilla::Telemetry::E10S_STATUS, status);
   if (Preferences::GetBool("browser.enabledE10SFromPrompt", false)) {
     mozilla::Telemetry::Accumulate(mozilla::Telemetry::E10S_STILL_ACCEPTED_FROM_PROMPT,
                                     gBrowserTabsRemoteAutostart);
   }
   if (prefEnabled) {