By far the most useful plugin I've added to Firefox in the past half-year or so:
SmartSearch adds a context menu item that allows you to send selected text (or whatever word you right-clicked on) to any one of the links in the Bookmarks 'Quick Searches' folder (Bookmarks->Organize Bookmarks->Bookmarks Menu->Quick Searches). I had to create this folder myself, because at some point I short-sightedly deleted it as being useless.
The SmartSearch page on quicksearches has a number of handy links such as a dictionary, thesaurus, IMDB, amazon, netcraft, whois, wayback archive, and google maps. I added a few more for google images and some source code pages:
Google Images Quicksearch
Google Code Quicksearch
Krugle Code Quicksearch (requires JS)
Codease FullText Query
Obviously many more can come in handy, and the code search engines can be trimmed down to just one or two (probably Koders and Codease).
The good news is that new Quicksearches are simple to create: navigate to the search page, right-click on the search form, select 'Add a keyword for this search', invent some useful keyword, and use the Quick Searches folder for the 'Create in' location.