Issue 3879 - CSS property filters ignore third-party stylesheets in Chrome but not in Firefox

chrome/content/cssProperties.js

 function splitSelector(selector)
 {
   if (selector.indexOf(",") == -1)
     return [selector];
 
   var selectors = [];
   var start = 0;
   var level = 0;
   var sep = "";
 
@@ skipping 38 matching lines @@
     {
       var property = style.item(i);
       var value    = style.getPropertyValue(property);
       var priority = style.getPropertyPriority(property);
       styles.push(property + ": " + value + (priority ? " !" + priority : "") + ";");
     }
     styles.sort();
     return styles.join(" ");
   },
 
+  isSameOrigin: function(stylesheet)
+  {
+    try
+    {
+      return new URL(stylesheet.href).origin == this.window.location.origin;
+    }
+    catch (e)
+    {
+      // Invalid URL, assume that it is first-party.
+      return true;
+    }
+  },
+
   findSelectors: function(stylesheet, selectors)
   {
+    // Explicitly ignore third-party stylesheets to ensure consistent behavior
+    // between Firefox and Chrome.
+    if (!this.isSameOrigin(stylesheet))
+      return;
+
     var rules = stylesheet.cssRules;
     if (!rules)
       return;
 
     for (var i = 0; i < rules.length; i++)
     {
       var rule = rules[i];
       if (rule.type != rule.STYLE_RULE)
         continue;
 
@@ skipping 49 matching lines @@
   apply: function()
   {
     if (this.patterns.length > 0)
     {
       var document = this.window.document;
       this.addSelectors(document.styleSheets);
       document.addEventListener("load", this.onLoad.bind(this), true);
     }
   }
 };