If you're a hardcore Web browsing fiend (no, not that kind of hardcore), then the kinds of add-ons that likely interest you are the ones that enable you surf as fast as humanly possible. But trawling site, after site, after site is often limited by both your connection speed and the speed of the site/server you're accessing, not to mention a few other little factors here and there.
I can't do much to help you with that via a simple Firefox add-on. However, I can assist you in finding information faster on the Web. Specifically, I can show you how to access previews for interesting links before you take the time (and resources) to open them up in a new tab, scan the page, and close them (or use them to continue about your way.) This might not sound like much of a benefit to one who's used to dumping a ton of new tabs based on links throughout a Web site. But hear me out--I've used CoolPreviews and it's a pretty sweet deal.
Here's how it works. By default, CoolPreviews appends a little icon next to a link once you've hovered your mouse over said link for a second or so. Click on the icon and up pops a lightbox-style frame with a preview of said link's final destination for you to peruse. You can send said link to a temporary sidebar on the right-hand side of your screen--the "stack"--or you can use the included icons to toss it back to a tab in your main browser or email it to a friend.
I've found that the best use of CoolPreviews comes when you edit the configuration screen to force the aforementioned preview window to pop up whenever you hover your mouse over a link, period. It's super-speedy to browse the Web in this fashion by hovering and hitting the escape key for sites you don't care to read any more about!
Maximum PC picks one new Firefox add-on as its favorite of the week each... week. Have a nifty extension that you can't live without? Twitter David Murphy @acererak with your latest suggestions.