Issue 4067 - Make filter "keys" generated for element hiding filters numeric

lib/elemHide.js

Index: lib/elemHide.js
===================================================================
--- a/lib/elemHide.js
+++ b/lib/elemHide.js
@@ -26,17 +26,17 @@ var {IO} = require("io");
 var {Prefs} = require("prefs");
 var {ElemHideException} = require("filterClasses");
 var {FilterNotifier} = require("filterNotifier");
 
 /**
  * Lookup table, filters by their associated key
  * @type Object
  */
-var filterByKey = Object.create(null);
+var filterByKey = [];
 
 /**
  * Lookup table, keys of the filters by filter text
  * @type Object
  */
 var keyByFilter = Object.create(null);
 
 /**
@@ -111,17 +111,17 @@ var ElemHide = exports.ElemHide =
     styleURL = Services.io.newFileURI(styleFile).QueryInterface(Ci.nsIFileURL);
   },
 
   /**
    * Removes all known filters
    */
   clear: function()
   {
-    filterByKey = Object.create(null);
+    filterByKey = [];
     keyByFilter = Object.create(null);
     filtersByDomain = Object.create(null);
     knownExceptions = Object.create(null);
     exceptions = Object.create(null);
     ElemHide.isDirty = false;
     ElemHide.unapply();
   },
 
@@ -142,22 +142,17 @@ var ElemHide = exports.ElemHide =
       exceptions[selector].push(filter);
       knownExceptions[filter.text] = true;
     }
     else
     {
       if (filter.text in keyByFilter)
         return;
 
-      let key;
-      do {
-        key = Math.random().toFixed(15).substr(5);
-      } while (key in filterByKey);
-
-      filterByKey[key] = filter;
+      let key = filterByKey.push(filter) - 1;
       keyByFilter[filter.text] = key;
 
       if (usingFiltersByDomain)
       {
         let domains = filter.domains || defaultDomains;
         for (let domain in domains)
         {
           let filters = filtersByDomain[domain];
@@ -329,27 +324,31 @@ var ElemHide = exports.ElemHide =
   _generateCSSContent: function*()
   {
     // Grouping selectors by domains
     let domains = Object.create(null);
     let hasFilters = false;
     for (let key in filterByKey)
     {
       let filter = filterByKey[key];
+      let selector = filter.selector;
+      if (!selector)
+        continue;
+
       let domain = filter.selectorDomain || "";
 
       let list;
       if (domain in domains)
         list = domains[domain];
       else
       {
         list = Object.create(null);
         domains[domain] = list;
       }
-      list[filter.selector] = key;
+      list[selector] = key;
       hasFilters = true;
     }
 
     if (!hasFilters)
       throw Cr.NS_ERROR_NOT_AVAILABLE;
 
     function escapeChar(match)
     {