I've been using the SmarterFox Firefox add-on (Windows | Mac) for awhile now. It's by far one of the most varied productivity add-ons for Firefox that I've seen. Its clever tricks to speed up searching, browsing, downloading, and even copy and paste are good news for just about anyone whose job description contains the word "Web."
I've put together a SmarterFox slideshow to walk you through the substantial features. Without giving away too much, SmarterFox facilitates faster search through multiple channels, including the Firefox Awesome Bar, the context menu, and Wikipedia enhancements. It also helps launch Web sites faster, and has a novel way of dealing with pagination for sites with multipage articles, and for shopping sites displaying pages of results. Check out the gallery to see exactly how these features work.