There are some really great add-ons out there for firefox. I'm sure there are ones i could use and just don't know about.
Im using the following add-ons with firefox 3:
Blocks 99% of all ads seen on webpages. no annoying pron side bars for me. Also gives the option to block other images and frames. Its a must have.
AdBlock Filterset.G updater
Connects to a server with an updated list of ads to block everyday. Works in conjunction with Adblock Plus.
Adbplock Element Hiding
used in conjunction with Ablock plsu. Allows you to block those annoying google text adds.
Tab Mix Plus
A must for firefox users. Allows you to customise how your tabs work. Really powerful add-on.
Fixes problems in urls like hxxp:// will be changed to http:// or con to com or oeg to org. Very handy if ppl are hiding links by spelling them wrong.
Smart bookmarks Bar
This removes the names from the bookmarks in the bookmarks toolbar leaving only the webside icon[like blue G for google].
When you hover over the link the name slides out. Very handy for fitting about 80 links in the bookmarks toolbar.
Use tamperdata to view and modify HTTP/HTTPS headers and post parameters. [Do not use if you dont know how!]
Dictionary Lookup Extension
Select a word and right click and go down to define and the defintion will slide out. no need to load other pages.
Irc program inside firefox. Will open link like [irc://irc.rizon.net/animea] I prefer mirc though.