Adblock 0.5.2.055 (google for this version, not the one on Mozilla's site) - First extention you need to install.
Blockfall- Tetris. 'Nuff said.
Cutemenus 2 - Puts icons on most standard menu items.
Delicious Delicacies - Cookies are delicious delicacies.
downTHEMall! - Found it useful once when I had 100 links I wanted to download at once.
FasterFox - Turbocharges Firefox's file transfering, upping limits on transfers.
Favicon Picker - Lets you fix broken icons in favorites.
FirefoxView - Installs an IE extention "View in Firefox" context menu item.
Google Toolbar For Firefox -
http://toolbar.google.com/firefox/ - I like the Google Toolbar. To each his own, though.
Greasemonkey - Great concept, I have yet to make any scripts for it (or find any useful ones, any suggestions?)
IE Tab - Allows embeding of an IE window in a tab for IE browsing through Firefox for selective sites, ie Windows Update. Needs a bit of tweaking though.
Menu Editor - Very useful. You can move menu items, and hide/show them, so useless extention items you couldn't hide are now hidable, not to mention normal menu options. Also allows for hiding of Go, Edit, Help, etc.
Mines - This > winmine.exe ^ 2
Nightly Tester Tools - Force any extention to install regardless of it's Firefox version compatability.
Nuke Anything - Fun. Get R.I.P. for permanent changes.
Opera View - Like IE View but.... for Opera! I use this to test my webpage designs.
RadialContext - I don't use it anymore but it's nicer looking and faster than the menu once you get used to it.
Remove It Permanently - Takes care of the few things AdBlock can't handle.
ScrapBook - Save and manage HTML snippets. I should use this more often... useful if you want to save a piece of text, and you don't have to File SaveAs and then go hunting later.
Screen Grab! - Haven't tried it, but it should allow you to take a screenshot of a webpage (ie the whole thing, it'll scroll around and piece it together).
Scribe - Useful for the ISB.
Adds some save/open features to forms.
Show Image - The usefullness-to-profile ratio of this is through the roof... that is, it's useful when my freaking proxy breaks some images and makes it so I don't have to reload the whole page.
SmoothWheel - A bit better than the standard wheel mouse scrolling.
Sort Extentions - I hate Slim Extention's "slim" mode, so this is nice. The sorting is freaking slow though.
Tab Thumbnail Preview - Emulates a forthcoming IE7 feature. It's not as accessable as it should be tho.
Tabs Menu - Adds a menu with all your tabs, sorta like a Window menu for tabs.
Tiny Menu - Basically Compact Menu for 1.5. Frees up a bit of toolbar real estate on your menu bar.
User Agent Switcher - For those few sites that still think Firefox is a failure, all hail King IE, blah blah.
View Formatted Source - Tidy up View Source. Makes it easier to browse source code.
Web Developer - Toolbar that greatly helps with HTML editing.