I don't run too many Firefox plugins, but I really love the ones I do use. Here's a run-down. The title of each section will link directly to the plugin on the mozilla site.
Even if you don't use Firefox plugins at all, I recommend giving NoScript a try. From the NoScript website:
Security Reality Check by ax0n: Switching between multiple public proxies every 30 seconds might seem like a good idea for making yourself harder to track, but it also dramatically increases the number of places your traffic goes. You leave more footprints in more places, which could actually make it easier to track something back to you, even if it's harder to figure out everything you did.
Ubiquity is a command line interface to Mozilla Firefox. This allows you to create small, re-usable custom functions and subscribe to third party functions. If, like me, you find yourself willing and ready to script-automate the repetitive things in life, You'll probably love Ubiquity.
I'm paranoid by nature, and NoScript is the only plugin I leave enabled all the time. The rest of these I will only enable when I will need them. I'm leery about using a lot of Greasemonkey scripts, and don't really like loading my browser with dozens of add-ons. Do you have some must-have favorites that I'm really missing out on?