By I have recently came across arguably the best extension ever for Firefox. If you've ever used Opera there's a chance you'll have seen Speed Dial; which is a feature in Opera 9.2 which allows you to transform your top nine sites into visual favorites that are immediately available in any new tab. Now, Firefox has it's own version of Speed Dial and it works just like the Opera version. The really nice thing is that you can set it up as a start page in Firefox and then browse from there. I've got it set up with My Yahoo, Google, eBay, Facebook, MySpace and Online Banking sites along with a couple of others and can easily just click on an image to instantly go straight to that site.
I can also recommend a couple of other nifty Firefox extensions. BugMeNot tries to take the pain out of accessing free content on sites that ask you to register and Foxmarks is a bookmark synchroniser that is handy for keeping track of your bookmarks on multiple computers.
There is also firebug which is for web deveolpment and it tells you where errors are
https://addons.mozilla.org/en-US/firefox/addon/1843