--- a/build/valgrind/mach_commands.py
+++ b/build/valgrind/mach_commands.py
@@ -96,16 +96,17 @@ class MachCommands(MachCommandBase):
                                      locations=locations)
 
             firefox_args = [httpd.get_url()]
 
             env = os.environ.copy()
             env['G_SLICE'] = 'always-malloc'
             env['MOZ_CC_RUN_DURING_SHUTDOWN'] = '1'
             env['MOZ_CRASHREPORTER_NO_REPORT'] = '1'
+            env['MOZ_DISABLE_NONLOCAL_CONNECTIONS'] = '1'
             env['XPCOM_DEBUG_BREAK'] = 'warn'
 
             env.update(self.extra_environment_variables)
 
             outputHandler = OutputHandler(self.log)
             kp_kwargs = {'processOutputLine': [outputHandler]}
 
             valgrind = 'valgrind'

--- a/testing/gtest/rungtests.py
+++ b/testing/gtest/rungtests.py
@@ -91,16 +91,17 @@ class GTests(object):
         env["MOZ_XRE_DIR"] = self.xre_path
         env["MOZ_GMP_PATH"] = os.pathsep.join(
             os.path.join(self.xre_path, p, "1.0")
             for p in ('gmp-fake', 'gmp-fakeopenh264')
         )
         env["XPCOM_DEBUG_BREAK"] = "stack-and-abort"
         env["MOZ_CRASHREPORTER_NO_REPORT"] = "1"
         env["MOZ_CRASHREPORTER"] = "1"
+        env["MOZ_DISABLE_NONLOCAL_CONNECTIONS"] = "1"
         env["MOZ_RUN_GTEST"] = "1"
         # Normally we run with GTest default output, override this to use the TBPL test format.
         env["MOZ_TBPL_PARSER"] = "1"
 
         if not mozinfo.has_sandbox:
             # Bug 1082193 - This is horrible. Our linux build boxes run CentOS 6,
             # which is too old to support sandboxing. Disable sandbox for gtests
             # on machines which don't support sandboxing until they can be

--- a/toolkit/xre/nsAppRunner.cpp
+++ b/toolkit/xre/nsAppRunner.cpp
@@ -4902,65 +4902,72 @@ bool XRE_Win32kCallsAllowed() {
     default:
       return true;
   }
 }
 #endif
 
 // If you add anything to this enum, please update about:support to reflect it
 enum {
-  kE10sEnabledByUser = 0,
+  // kE10sEnabledByUser = 0, removed when ending non-e10s support
   kE10sEnabledByDefault = 1,
   kE10sDisabledByUser = 2,
   // kE10sDisabledInSafeMode = 3, was removed in bug 1172491.
   // kE10sDisabledForAccessibility = 4,
   // kE10sDisabledForMacGfx = 5, was removed in bug 1068674.
   // kE10sDisabledForBidi = 6, removed in bug 1309599
   // kE10sDisabledForAddons = 7, removed in bug 1406212
   kE10sForceDisabled = 8,
   // kE10sDisabledForXPAcceleration = 9, removed in bug 1296353
   // kE10sDisabledForOperatingSystem = 10, removed due to xp-eol
 };
 
-const char* kForceEnableE10sPref = "browser.tabs.remote.force-enable";
-const char* kForceDisableE10sPref = "browser.tabs.remote.force-disable";
-
 namespace mozilla {
 
 bool BrowserTabsRemoteAutostart() {
   if (gBrowserTabsRemoteAutostartInitialized) {
     return gBrowserTabsRemoteAutostart;
   }
   gBrowserTabsRemoteAutostartInitialized = true;
 
   // If we're in the content process, we are running E10S.
   if (XRE_IsContentProcess()) {
     gBrowserTabsRemoteAutostart = true;
     return gBrowserTabsRemoteAutostart;
   }
 
-  bool optInPref = Preferences::GetBool("browser.tabs.remote.autostart", true);
+#if defined(MOZILLA_OFFICIAL) && MOZ_BUILD_APP_IS_BROWSER
+  bool allowSingleProcessOutsideAutomation = false;
+#else
+  bool allowSingleProcessOutsideAutomation = true;
+#endif
+
   int status = kE10sEnabledByDefault;
-
-  if (optInPref) {
-    gBrowserTabsRemoteAutostart = true;
+  // We use "are non-local connections disabled" as a proxy for
+  // "are we running some kind of automated test". It would be nicer to use
+  // xpc::IsInAutomation(), but that depends on some prefs being set, which
+  // they are not in (at least) gtests (where we can't) and xpcshell.
+  // Long-term, hopefully we can make tests switch to environment variables
+  // to disable e10s and then we can get rid of this.
+  if (allowSingleProcessOutsideAutomation ||
+      xpc::AreNonLocalConnectionsDisabled()) {
+    bool optInPref =
+        Preferences::GetBool("browser.tabs.remote.autostart", true);
+
+    if (optInPref) {
+      gBrowserTabsRemoteAutostart = true;
+    } else {
+      status = kE10sDisabledByUser;
+    }
   } else {
-    status = kE10sDisabledByUser;
-  }
-
-  // Uber override pref for manual testing purposes
-  if (Preferences::GetBool(kForceEnableE10sPref, false)) {
     gBrowserTabsRemoteAutostart = true;
-    status = kE10sEnabledByUser;
   }
 
   // Uber override pref for emergency blocking
-  if (gBrowserTabsRemoteAutostart &&
-      (Preferences::GetBool(kForceDisableE10sPref, false) ||
-       EnvHasValue("MOZ_FORCE_DISABLE_E10S"))) {
+  if (gBrowserTabsRemoteAutostart && EnvHasValue("MOZ_FORCE_DISABLE_E10S")) {
     gBrowserTabsRemoteAutostart = false;
     status = kE10sForceDisabled;
   }
 
   gBrowserTabsRemoteStatus = status;
 
   return gBrowserTabsRemoteAutostart;
 }