--- a/browser/base/content/contentSearchUI.js
+++ b/browser/base/content/contentSearchUI.js
@@ -77,16 +77,19 @@ ContentSearchUIController.prototype = {
   // if it's set when the suggestions table is actually opened.
   _pendingOneOffRefresh: undefined,
 
   get defaultEngine() {
     return this._defaultEngine;
   },
 
   set defaultEngine(engine) {
+    if (this._defaultEngine && this._defaultEngine.icon) {
+      URL.revokeObjectURL(this._defaultEngine.icon);
+    }
     let icon;
     if (engine.iconBuffer) {
       icon = this._getFaviconURIFromBuffer(engine.iconBuffer);
     }
     else {
       icon = this._getImageURIForCurrentResolution(
         "chrome://mozapps/skin/places/defaultFavicon.png");
     }
@@ -863,16 +866,20 @@ ContentSearchUIController.prototype = {
       if (engine.iconBuffer) {
         uri = this._getFaviconURIFromBuffer(engine.iconBuffer);
       }
       else {
         uri = this._getImageURIForCurrentResolution(
           "chrome://browser/skin/search-engine-placeholder.png");
       }
       img.setAttribute("src", uri);
+      img.addEventListener("load", function imgLoad() {
+        img.removeEventListener("load", imgLoad);
+        URL.revokeObjectURL(uri);
+      });
       button.appendChild(img);
       button.style.width = buttonWidth + "px";
       button.setAttribute("title", engine.name);
 
       button.engineName = engine.name;
       button.addEventListener("click", this);
       button.addEventListener("mousemove", this);