One toolbar that's not included in Chris's Firefox overview is one released a few minutes ago.
Clusty's Toolbar for Firefox offers up what you've come to expect from search toolbars along with a context-sensitive searching option called "Clusty Clips." This feature allows the searcher to access definitions and encyclopedia entries (via Wikipedia) and run searches without having to click off of a web page.
Finally, Michael Abramowitz from Yahoo, offers up a Firefox/Yahoo extension on the Yahoo Search Blog.
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