Issue 3964 - Fix race condition, CSS property filters applying too early

lib/child/cssProperties.js

Index: lib/child/cssProperties.js
===================================================================
--- a/lib/child/cssProperties.js
+++ b/lib/child/cssProperties.js
@@ -57,17 +57,17 @@
 
     let onContentWindow = (subject, topic, data) =>
     {
       if (!(subject instanceof Ci.nsIDOMWindow))
         return;
 
       let onReady = event =>
       {
-        subject.removeEventListener("DOMContentLoaded", onReady);
+        subject.removeEventListener("load", onReady);
         let handler = new scope.CSSPropertyFilters(subject, selectors =>
         {
           if (selectors.length == 0)
             return;
 
           addUserCSS(subject, selectors.map(
             selector => selector + "{display: none !important;}"
           ).join("\n"));
@@ -76,17 +76,17 @@
         // HACK: The content script just calls ext.backgroundPage.sendMessage
         // without indicating which window this is about. We'll store the window
         // here because we know that messaging happens synchronously.
         senderWindow = subject;
         handler.load(() => handler.apply());
         senderWindow = null;
       };
 
-      subject.addEventListener("DOMContentLoaded", onReady);
+      subject.addEventListener("load", onReady);
     };
 
     Services.obs.addObserver(onContentWindow, "content-document-global-created",
         false);
     onShutdown.add(() =>
     {
       Services.obs.removeObserver(onContentWindow,
           "content-document-global-created");