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

chrome/content/cssProperties.js

Index: chrome/content/cssProperties.js
===================================================================
--- a/chrome/content/cssProperties.js
+++ b/chrome/content/cssProperties.js
@@ -51,18 +51,36 @@ CSSPropertyFilters.prototype = {
       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)