I just saw this article on Sphinn about '20+ Must-Have FireFox Search Plugins for SEOs' by Ann Smarty over at Search Engine Journal. This is a pretty comprehensive list of search engine plugins to add to the toolbar.
I am going to be adding a few from this list, particularly:
1. Whois Look-Up
2. Google unpersonalized search
3. Del.icio.us
4. Smart Page Rank
5. Quantcast
6. TinyURL