Firefox should implement a way to change the default (address bar) search engine inside it's own interface.
A lot of search engines are hijacking the default search engine. The only way that I know to set it back to google (or whatever you need) is by changing the settings manually. And I must say that is a pain. I really love Firefox 4, but this part is REALLY annoying. You should release a hotfix ASAP to implement a way to restore the default search engine. Maybe make the default search engine on the search bar works for the address bar too?
Anyway, fix this one. It's been around for too long.