I keep coming back to Firefox as my browser of choice because of the plethora of extensions available for it (though there are only a few I use regularly). I was going to write of just one here, but now, I think I’ll just list those I find most useful, and why (in no particular order).
1. Adblock Plus: Lets you block dross on the net. Rightclick and banish a banner/server forever.
2. Adblock Filterset.G Updater: This retrieves an updated set of filters which work in Adblock and AdblockPlus. You still need to do your own blocking but this gives a head start.
3. Temporary Inbox: Gives you a disposable email address which is good for 6 hours. Excellent for signing up at sites which you know are going to generate tons of spam, just so you can get to see what you want to. Not good if your email address is needed in any ongoing way.
4. gMail Manager: Puts a notification area into your statusbar and allows checking of multiple gMail accounts, unlike most which check only one. Also gives you snippets of available mail.
5. Scrapbook: Collecting information has never been so easy (except with the iHarvest IE extension which is old and no longer supported). Make your own database of collected info. Excellent for researchers.
6. Videodownloader: I confess to not using this much, but if I want to, I can get videos of most kinds from most sites.
7. BBCodeXtra: If you frequent forums, you’ll find most of them use BBCode or a version of it, to display such things as bold type, different colour fonts etc. A right click in the window and selection from the context sensitive menu makes it easy to use. Works for Invision and vBulletin that I know of, and probably for phpBB as well, but I havent tried it there myself.
I have a few more extensions but these are my favourites 🙂