Adblock Plus 1.0.1 – Perfect show of what Firefox developers are capable of. Subscribe to EasyList, and AdBlockRules.org. Trust me, worth it.
AutoPager 0.4.1.1 – Recently turned on to this by Andy. Automatically paginates things like search results or blogs that have multiple pages.
Better Gmail 2 0.7.3.2
Better GReader 0.5.2
Better Lifehacker 0.4 – The Better series, started from Lifehacker, just plain rocks, especially the flagship Better GMail 2. It’s Greasemonkey scripts rolled into an extension. Makes GM easy.
Delicious Bookmarks 2.1.018 – If you use delicious, this is for you.
Download Statusbar 0.9.6.4 – Keeps all your downloads handy at the bottom of the browser rather than a second small window. Keeps things nice and tidy on the taskbar
DownThemAll! 1.1.1 – Ever have to download all the PDF manuals on one page? Get this and it will cure your rapid onslaught of cursing.
Google Gears 0.5.4.2 – Get this to take Google offline.
Greasefire 1.0.3 – Finds Greasemonkey scripts for pages you are on.
Greasemonkey 0.8.20090123.1 – The end-all extension. You can take any site and modify it with javascript to make it do whatever you want.
InfoLister 0.10 – The cool extension that lets you print off your list of extensions to paste on your website to dote on .
Novell Moonlight 1.0 – Microsoft’s Silverlight ported to Linux. Not everyone needs this, but for those running Ubuntu, it’s wonderful.
RetailMeNot 2.2 – Trying to shop cheaply? Install this, it will alert you to coupon codes for the site. Great to save a couple of bucks.
Tab Mix Plus 0.3.7.3 – Tabs, the way you want them. This extension can do anything you want with tab actions or display of the tabs.
Ubuntu Firefox Modifications 0.6 – Firefox standard in Ubuntu. Makes things work all shiny.
Woot Watcher 1.0.6 – For Wootheads like me, this puts it down in the corner for you. And will pop it up at midnight if you’re awake.

What about you, what do you use?