Issue 4221 - Only forward one elemhideupdate message per update batch to the content process

lib/elemHideFF.js

Index: lib/elemHideFF.js
===================================================================
--- a/lib/elemHideFF.js
+++ b/lib/elemHideFF.js
@@ -18,18 +18,31 @@
 "use strict";
 
 let {port} = require("messaging");
 let {ElemHide} = require("elemHide");
 let {FilterNotifier} = require("filterNotifier");
 let {FilterStorage} = require("filterStorage");
 let {Prefs} = require("prefs");
 let {Policy} = require("contentPolicy");
+let {Utils} = require("utils");
 
-FilterNotifier.on("elemhideupdate", () => port.emit("elemhideupdate"));
+let isDirty = false;
+FilterNotifier.on("elemhideupdate", () =>
+{
+  // Notify content process asynchronously, only one message per update batch.
+  if (!isDirty)
+  {
+    isDirty = true;
+    Utils.runAsync(() => {
+      isDirty = false;
+      port.emit("elemhideupdate")
+    });
+  }
+});
 
 port.on("getSelectors", () => ElemHide.getSelectors());
 
 port.on("elemhideEnabled", ({frames, isPrivate}) =>
 {
   if (!Prefs.enabled)
     return {enabled: false};