>>542
You should get Adblock plus and NoScript for security measures.

If you ever plan on using IRC then I think that Chatzilla might be a good addon for it, but since I don't ever use it, I'm not entirely sure about it.

If you ever plan on using any file sharing sites (mediafire, megaupload, rapidshare, and a couple of others) Skipscreen works really well and makes using those sites a bit easier.

Also, Flashgot is a pretty good tool to use to download stuff with.