More extensions that I use:
Disable Targets for Downloads: There are a number of times when I try to download something and Firefox opens a new window/tab. This extension simply removes that annoyance.
Adblock: Remove those ads that annoy you with this extension. This has been nice to use for those ads that play sounds. Remove just the single ad or completely block the server it comes from.
Tabbrowser Preferences: Tabbed browsing saves me from having a large number of windows open at the same time, but I never cared for the positioning of the tabs. I use this extension to have the tabs at the bottom of the screen.
PwdHash: It's difficult to keep track of passwords, especially when you have dozens of them, whether they be for your operating system, a special program, or a website. As such, most people tend to use the same password in multiple places, which isn't the wisest idea if someone figures out that password. PwdHash will automatically make a more secure password for a site while you continue to use that easy to remember password. Let the extension description and the PwdHash site explain it more. My only complaint about this is that I would like a way to know which sites I used PwdHash with - granted, I should just go through all sites and change my passwords to use this...