About.com: Top 10 Firefox extensions
A collection of some of the most popular and useful general Firefox extensions.
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.