Cool, some suggestions Rated 4 out of 5 stars
That's a cool add-on. In fact, I was thinking to build a similar add-on, and would like to name it "smash", which means "searching mash-up", then I found this one.
1. If I add a new search engine that is not in the list, the tool cannot save the information. For example, I would like to add "www.live.com", after I quited Firefox, that setting was lost.
2. I think it is not necessary to allow in-page configuration (moving mouse to top for changing search engines). I think it's better to have a simple configuration interface for setting up which search engines to go to and have a "Ok" or "Save" button. It's a bit confusing for me to change search engine settings when I click the "a" button.
3. Maybe you want to put Microsoft (www.live.com) into the tool.