This is a lifesaver if you've come from Chrome, which is much better for search engine customization. It works very nicely, and unlike another add-on I tried, it will find the appropriate favicon for the search engine on its on.

Only real flaw is after you add a search engine, when you add the next it will by default continue using the last favicon. You have to always delete the current custom favicon url before entering the next search string if you want the write icon.

If you add a different url it should find the proper favicon. Short of that, there should be a "clear" button so I don't have to select and delete.

Also it won't let you add a nickname, but I'm assuming that's a limitation of Firefox.