fix #2256

platform/firefox/vapi-client.js

@@ -510,6 +510,20 @@ if ( self.injectCSS ) {
 
 /******************************************************************************/
 
+// https://bugzilla.mozilla.org/show_bug.cgi?id=444165
+// https://github.com/gorhill/uBlock/issues/2256
+//   Not the prettiest solution, but that's the safest/simplest I can think
+//   of at this point. If/when bugzilla issue above is solved, we will need
+//   version detection to decide whether the patch needs to be applied.
+
+vAPI.iframeLoadEventPatch = function(target) {
+    if ( target.localName === 'iframe' ) {
+        target.dispatchEvent(new Event('load'));
+    }
+};
+
+/******************************************************************************/
+
 // No need to have vAPI client linger around after shutdown if
 // we are not a top window (because element picker can still
 // be injected in top window).

src/js/contentscript.js

@@ -947,6 +947,8 @@ vAPI.domCollapser = (function() {
             netSelectorCacheCountMax = response.netSelectorCacheCountMax,
             aa = [ null ],
             request, key, entry, target, value;
+        // https://github.com/gorhill/uBlock/issues/2256
+        var iframeLoadEventPatch = vAPI.iframeLoadEventPatch;
         // Important: process in chronological order -- this ensures the
         // cached selectors are the most useful ones.
         for ( var i = 0, ni = requests.length; i < ni; i++ ) {
@@ -980,6 +982,7 @@ vAPI.domCollapser = (function() {
                     selectors.push(request.tag + '[' + request.attr + '="' + value + '"]');
                     netSelectorCacheCount += 1;
                 }
+                if ( iframeLoadEventPatch ) { iframeLoadEventPatch(target); }
             }
         }
         if ( selectors.length !== 0 ) {
@@ -1030,9 +1033,6 @@ vAPI.domCollapser = (function() {
         }
     };
 
-    // If needed eventually, we could listen to `src` attribute changes
-    // for iframes.
-
     var add = function(target) {
         var tag = target.localName;
         var prop = src1stProps[tag];