Issue 6437 - Skip DOM-only patterns on initial load

lib/content/elemHideEmulation.js

Index: lib/content/elemHideEmulation.js
===================================================================
--- a/lib/content/elemHideEmulation.js
+++ b/lib/content/elemHideEmulation.js
@@ -448,20 +448,28 @@
     // one of the patterns.
     return getCachedPropertyValue(
       this, "_dependsOnCharacterData",
       () => this.selectors.some(selector => selector.dependsOnCharacterData)
     );
   }
 };
 
-function filterPatterns(patterns, {stylesheets, mutations})
+function filterPatterns(patterns, {stylesheets, mutations}, document)
 {
   if (!stylesheets && !mutations)
+  {
+    // On initial load, skip DOM-only patterns if it's the top frame, because
+    // there's usually nothing interesting to see and we can respond to any
+    // mutations later.
+    if (typeof top != "undefined" && document.defaultView == top)
+      return patterns.filter(pattern => pattern.dependsOnStyles);
+
     return patterns.slice();
+  }
 
   return patterns.filter(
     pattern => (stylesheets && pattern.dependsOnStyles) ||
                (mutations && pattern.dependsOnDOM)
   );
 }
 
 function shouldObserveAttributes(patterns)
@@ -572,17 +580,18 @@
    *    made reprocessing necessary. This parameter shouldn't be passed in for
    *    the initial processing, the entire document will be considered
    *    then and all rules, including the ones not dependent on the DOM.
    * @param {function} [done]
    *    Callback to call when done.
    */
   _addSelectors(stylesheets, mutations, done)
   {
-    let patterns = filterPatterns(this.patterns, {stylesheets, mutations});
+    let patterns = filterPatterns(this.patterns, {stylesheets, mutations},
+                                  this.document);
 
     let selectors = [];
     let selectorFilters = [];
 
     let elements = [];
     let elementFilters = [];
 
     let cssStyles = [];