NoScript-Needs to be built in and defaulted on. But then I guess we'd lose a lot of business if that happened.
Adblock Plus-This one will spoil you. I can't stand to use a browser without this now. It always freaks me out when I realize just how much crap this filters out of my life.
Greasemonkey-There are some people doing really cool stuff with this. From general tweaks to the way the browser works to scripts that make individual websites much more functional.
Xmarks Sync-Absolutely love it. That site I bookmarked last night at home is automatically in the same place on my portable version when I open it up the next day. And when my main box crashed a couple of weeks ago, I was able to install FF on my test box and have my bookmarks back in minutes. I think this is a good one for everyone to set up for their customers. Helps prevent them from losing several years of saved bookmarks. Imagine what a hero you'll be when you magically restore all of the bookmarks they though they lost when their system crashed.
Tab Mix Plus-Somewhat redundant nowadays, but still a lot of useful tab tweaks. But if you just want to add a certain feature to your tabs, you better off looking for a plug-in that does just that.
WoT-My personal feeling is that NoScript and ABP make this irrelevant, But I guess it's useful if you want to leave scripts and ads on.
Download Statusbar-Addresses fact that the stock download function sucks. There are several variants and I personally use Download Manager Tweak, but there's probably something that's better than the default that suits your persona needs.
LastPass Password Manager-I see the appeal, but I don't use this or the Xmarks password function. I prefer KeePass with the password file stored in My Dropbox account.
FoxyTunes-Love it and it works with any music player worth using.
WebMail Notifier-At the office it's fairly useless. But at home, where there's a dozen or so email accounts on 3 different services, it's brilliant.
Extensions that need to be on the list:
Beef Taco (Targeted Advertising Cookie Opt-Out) Don't let the marketing people track you.
Context Search-allows you to use all of your installed search engines from the context menu and not just the one currently selected.
Quick Restart-As many have noted, FF does suffer from some bloat. And if I notice it dragging the system, I hit the button and restart. Easier than waiting for the bloat situation to fix itself.
URL Flipper-get through slide shows and the like with more ease as you have your own forward and back buttons that will always be in the same place.