First, I always install Adblock, even if I am just a guest on the computer. (Of course, I ask the owner permission, and demonstrate how to use it properly... but I always get them to at least let me install it if they already have Firefox, and they always decide to keep it.)
On the installs of Firefox that I have direct control over, I install Adsense Notifier... Partly because I just like numbers, and partly because it keeps me from being tempted to click on my own ads. If Google knows what computers I use, then there is no temptation to try and cheat the system, and all of my clicks are always legitimate.
On my "work" installations, I install FireFTP, Server Spy, and Web Developer, so that I can quickly troubleshoot any problems (not just on my own server; ask Cassie how useful it was that I could pull her CSS apart very quickly) and be able to connect to my own servers wherever I go. (It really isn't work if I like doing it, is it?)
I also use Firefox Portable on my portable hard drive, because I can never guarantee that I'll remain on the same computer. That way, I always have the tools that I'm used to very close at hand wherever I go, with vastly reduced fear of the flash memory going bad from too many write operations.
PortableApps.com - Portable software for USB drives | Your Digital Life, Anywhere