It adds much more control over the behaviour of your tabs. Tab Wheel Scroll fill my biggest need there, but lots of other customization in Tab Mix Plus that's super useful like selecting tabs by hovering or mouse gestures...
If you use Nagios for system monitoring (which you really should), then you'll want Nagios Checker which is a simple plugin which shows Nagios system status in your Firefox status box area. It's very flexible allowing you to disregard alerts based on system outages, acknowledgements, etc.
This plugin will generally block all ads on all sites.
With Add to Search Bar you can add the search functionality of any page to the search box so any site can become a "search provider".
Quicktext is most useful for short-hand email templates. I often use it to generate a couple different standard sigs, like on some emails I want my full business signature with contact information etc and other emails a personal sig or something abbreviated.
This is a very useful plugin which creates an address book to sync your contacts to / from Google. The nice thing is that it will mostly configure itself. It will create address books for each account you have setup in Thunderbird which accesses Google's servers.
Adds a lot of keyboard shortcuts to Thunderbird. I like being able to type "cc" at the start of a recipient line to change the line from a To: line to a CC: line.
Allows you to bounce emails. Old-school email hack which basically just resends the original email but to the recipient of your choosing. Unlike forwarding where you are just sending a copy of the email, bounced emails show the original sender and recipient information. Very useful for quickly redirecting email to the correct person.
See also: Wordpress MU
Turn your blog into an OpenID provider and use your own blog to login to other sites.
Add syntax highlighting to your blog posts. Essential for anyone who posts code snippets whether bash scripts or C programs.