Hrm... User Agent Switcher and "Things They Left Out" are the only extensions I really use. TTLO will let you set smooth scroll, and turn off that gawd-awful image resizing, among other things.

Or, you can get your hands dirty and learn how to disable/enable/change stuff without extensions. That's more fun. =D

Being a Firebird user/lover, I would suggest tabbed browser, all in one mouse gestures(got hooked on those with Opera) and the one that allows you to edit user option files. And welcome to Firebird, there is no going back now

MyIE2 would be better, I prefer Avant as an IE based browser. MyIE2 is just too busy for me... But I think Firebird will trump IE eventually, thought Opera would be the one, but I am not so sure of that now.

Ook! is slick and comes preinstalled these days so need to install that one. Ook! sticks your bookmarks in your context menu (something I've always wanted).

I always install Live HTTP Headers, so I can have a peak under the hood (since I code sites for a living, I'm always interested in what people have running and so on).

What else have I got: "text/plain", which allows you to use plain text urls as if they were links (too bad you have to select them first); and "copy image", which allows you to copy an image from a site to the clipboard from the context menu. Sounds slick, but haven't really used it though.

It's been a couple of days, but I'll throw my hat in here. Aside from the Tabbrowser Extension that everyone has already mentioned, I like Nuke Anything and Flash Click to View. Most of the time they work to restore sanity to browsing, the Flash ext. kind of sucks on all-Flash sites though.

Oh, and TagZilla is a cool little extension if you're using Thunderbird, too.