I know, I know, it's been done to death. Every tech blog on earth has a post with their favourite Firefox plugins. But I want to stick these out here, just so I feel like I'm part of the cool kids. So here are the best plugins I use, in no real order.
Web Developer Toolbar If you do anything with web development this is the bar to have. It is an unstopable juggernaut of features. You honestly just have to install it and go through every menu playing with what it can do. Amazing.
Fission It's not often I say I'm jealous of something on a macintosh, but this time I think I'll do it. The Safari browser has a really cool feature in that it's address bar is also it's progress bar. A big honkin progress bar. Fission is a nice copy of that functionality and one of my fave items, even though it breaks a few themes.
Foxmarks Bookmark Synchronizer Can you say awesome? Because you'll need that word to describe the coolness it feels with having the same, current bookmarks at home, work and on any machine you want. I even have it on my portable firefox so that I can take my bookmarks with me anywhere. Despite the mild security issue, it's still a great extension.
Adblock Industrial stength ad-removal for free. You have to love it and I do. If you get one extension on your install, get this one. It makes the internet less annoying.
Adblock Filterset G. Updater This is the perfect complement to Adblock, it arms and reloads Adblock with a great set of filters for smacking down ads.
Viamatic Foxpose This isn't one you hear about usually, but it can be handy for tab junkies. I like having lots of tabs and this provides an easy way to get to the right one quickly. It creates little screenshots of each tab and lets you click on the one you want to go to. Very cool.
FireFTP This is really a great extension. It lets you have a cross-platform fully functional FTP client with the same look and feel on any machine. Extremely useable, it's replaced gFTP and FileZilla in my life. I still use WinSCP for SSH connections though.
FoxyTunes This is a great plugin that adds a control bar for lots of players. I use it with Amarok and foobar2k. Very handy plugin, no more popping open another window just to change songs.
Aardvark While this extension takes some getting used to, I find it really handy. You can widen, thin, clear, remove and more any element you want with a keypress. Very hip.
MeasureIt MeasureIt lets you do just that, measure anything on the screen to pixel perfect numbers, handy for web developers and those optical illusion sites.
ColorZilla This color picker used to be one of my top tools, then it stopped working in Linux. I've tried the directions on the extension homepage, but to no avail. Still use this at work on my Windows machine though.
DownThemAll! DownThemAll! is a great multi-part file downloader/accelerator that can scrape entire pages. Very intelligent and easy to use.
Google Notebook Google Notebook is one of Google Labs beta services. A handy notebook for saving interesting tidbits you find across the net, this plugin lets you note faster and easier.
New Tab Homepage New Tab Homepage does exactly what it claims. When you open a new tab, it is set to your homepage, as opposed to the default empty page. Handy for those who keep a homepage of commonly used links.
Gmail Manager This little extension will constantly check your gmail account(s) and keep you up to date on their status. Very handy.
Thats all I've got for now, though I'll add to this list as I discover great new extensions.