A lot of us want to live in Firefox 2. But we also want to bend it to our will, to customize it so it's just right. Our visions of the ideal Firefox are different, though. The Web surfer will want one thing, for example, and the programmer another. So check out this Official Webware Firefox Top 10 List. Not of extensions. Of other lists.
Irfan Habib: My top 10 Firefox extensions
A thoughtful list for serious Firefox users. See also the comments to this post, which point to other good extensions.
Read/Write Web: Top 10 Firefox Web 2.0 add-ons
The focus is on effective browsing, bookmarks, and RSS. A good lineup if you explore the Web a lot and want to keep track of things you find.
The Simple Dollar: 10 Firefox plugs that save you money
Love the premise, even if it's stretched a bit thin in this list. This is an unconventional list of extensions that, one way or another, can impact your pocketbook.
CyberNet: 10 great Firefox extensions for tabbed browsing
You can push Firefox's breakout feature, tabbed browsing, to the limits if you use the extensions in this list. CyberNet also has a top 10 list of privacy and security extensions.
Quick Online Tips: Top 10 extensions to manage Firefox extensions
If you need this list, your Firefox setup has gotten way out of hand.
The Moleskin: Firefox extensions for Web development
If you're writing your own extensions, see this list.
Mozilla: 10 most popular Firefox add-ons
Maybe it's cheating to include this list, but if you want to know how everybody else is tweaking out their browser, don't neglect to go to the source.
Lifehacker: Top Firefox 2 config tweaks
Neither a list of extensions nor numbering 10, this is a tutorial, with excellent examples, on tweaking Firefox's many internal settings. A lot of the things you can do with these tweaks preclude the need for extensions, so it's worth checking out.