Extenstions:
Adblock is fantastic, right click on things, enter the * to work as a wildcard and you can block partial addresses from advertisers. Hurrah!
IE view is handy for the occasional website that relies on old badly coded script to make things happen that doesn't work with Firefox.
Foxytunes is good too, because you can control your media players through a tiny bar at the bottom of your browser. Cool!
I love 'All-in-one gestures' (in Mouse gestures) Hold the right mouse button and draw on the page to do things, draw up for a new tab is great, I use it constantly!
If you look in the Misc section you can get a 'homeland security threat indicator' to sit at the bottom of your browser window. Hurrah!
I like the 'Translate' extension. It saves me from copying stuff over into Babelfish manually. Perhaps not very useful if everything you see is in English.
Opine-It! 1.1 is a new one, but looks like it'll be great fun...
Oh, there's just loads. TabBrowser Preferendes hasn't been updated yet, but check back on the extenstions site occasionally for new releases. There really is loads of fun stuff in there, so have a browse around and try things out. I never thought that I'd find mouse gestures useful, but I use it constantly these days.
EDIT: IMPORTANT!
To make your life nice and easy, go to Bookmarks>Manage Bookmarks>File>Import to grab all your old bookmarks from IE and put them straight into Firefox. Handy