even better, just skip completely nsIContentPolicy for FF35+

platform/firefox/frameModule.js

@@ -72,7 +72,7 @@ const contentObserver = {
     contentBaseURI: 'chrome://' + hostName + '/content/js/',
     cpMessageName: hostName + ':shouldLoad',
     uniqueSandboxId: 1,
-    firefoxPost34: Services.vc.compare(Services.appinfo.platformVersion, '35.0') >= 0,
+    firefoxPre35: Services.vc.compare(Services.appinfo.platformVersion, '35.0') < 0,
 
     get componentRegistrar() {
         return Components.manager.QueryInterface(Ci.nsIComponentRegistrar);
@@ -101,6 +101,7 @@ const contentObserver = {
     register: function() {
         Services.obs.addObserver(this, 'document-element-inserted', true);
 
+        if ( this.firefoxPre35 ) {
             this.componentRegistrar.registerFactory(
                 this.classID,
                 this.classDescription,
@@ -114,23 +115,26 @@ const contentObserver = {
                 false,
                 true
             );
+        }
     },
 
     unregister: function() {
         Services.obs.removeObserver(this, 'document-element-inserted');
 
+        if ( this.firefoxPre35 ) {
             this.componentRegistrar.unregisterFactory(this.classID, this);
             this.categoryManager.deleteCategoryEntry(
                 'content-policy',
                 this.contractID,
                 false
             );
+        }
     },
 
     // https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Reference/Interface/nsIContentPolicy
     // https://bugzil.la/612921
     shouldLoad: function(type, location, origin, context) {
-        if ( this.firefoxPost34 || Services === undefined || !context ) {
+        if ( Services === undefined || !context ) {
             return this.ACCEPT;
         }