I use NoScript, AdBlock+, NewsFox, LeetKey and GMail Notifier on my laptop.
On the desktop box I have AdBlock+, User Agent Switcher, and Exploit-Me.
The last one is not for normal users and I don't recommend using it unless
you know what it does, have permission to test the security of a server, or
enjoy the idea of a cell mate named "Big Bubba" making you his wife.
I always Google an add-on to see what other people have to say. Or in the case
of the NewsFox add-on I just emailed the author(s) with my questions.
Current systems: WHAT OS, BackTrack-raw, PCLinuxOS, Peppermint OS 6, Kali Linux
and a custom Linux From Scratch server hosting a bunch of top secret stuff.