This are some of my frequence use Firefox Search Engines Add-ons.
Live.com - The Reason why I use this? MSN seach engine have some features that google don’t. For example you can search by IP (if you would like to know what website is living on this IP). Usually poeple use this to check what are the virtual hosts on the particular server. type this for example. ip:<x.x.x.x>
Wikipedia - Ya, I like WIKI. With Wikipedia you can easily find alots of useful info for almost everything under the sun ( sooner or later due to public contribution)

Youtube - ‘Youtubing’ become hobby of many peoples now day. If you often search movie clips over Youtube, you probably can try this.

Whois Lookup - How do you check on domain whois usual? through some whois website? Probably you can do it more faster through this Firefox search engine add-on.