shouldLoad needed only for FF34 and less (no need for sync messaging with e10s)

platform/firefox/frameModule.js

@@ -72,6 +72,7 @@ const contentObserver = {
     contentBaseURI: 'chrome://' + hostName + '/content/js/',
     cpMessageName: hostName + ':shouldLoad',
     uniqueSandboxId: 1,
+    firefoxPost34: Services.vc.compare(Services.appinfo.platformVersion, '35.0') >= 0,
 
     get componentRegistrar() {
         return Components.manager.QueryInterface(Ci.nsIComponentRegistrar);
@@ -126,13 +127,10 @@ const contentObserver = {
         );
     },
 
+    // https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Reference/Interface/nsIContentPolicy
     // https://bugzil.la/612921
     shouldLoad: function(type, location, origin, context) {
-        if ( Services === undefined ) {
-            return this.ACCEPT;
-        }
-
-        if ( !context ) {
+        if ( this.firefoxPost34 || Services === undefined || !context ) {
             return this.ACCEPT;
         }
 

platform/firefox/vapi-background.js

@@ -46,6 +46,7 @@ const {Services} = Cu.import('resource://gre/modules/Services.jsm', null);
 
 var vAPI = self.vAPI = self.vAPI || {};
 vAPI.firefox = true;
+vAPI.firefoxPre35 = Services.vc.compare(Services.appinfo.platformVersion, '35.0') < 0;
 
 /******************************************************************************/
 
@@ -1883,18 +1884,25 @@ vAPI.net.registerListeners = function() {
         pendingReq.tabId = tabWatcher.tabIdFromTarget(e.target);
     };
 
+    // https://github.com/gorhill/uMatrix/issues/200
+    // We need this only for Firefox 34 and less: the tab id is derived from
+    // the origin of the message.
+    if ( vAPI.firefoxPre35 ) {
         vAPI.messaging.globalMessageManager.addMessageListener(
             shouldLoadListenerMessageName,
             shouldLoadListener
         );
+    }
 
     httpObserver.register();
 
     cleanupTasks.push(function() {
+        if ( vAPI.firefoxPre35 ) {
             vAPI.messaging.globalMessageManager.removeMessageListener(
                 shouldLoadListenerMessageName,
                 shouldLoadListener
             );
+        }
         httpObserver.unregister();
     });
 };