Recommend Good Firefox extensions for developers? (I'll show you mine if you show me yours...)

[eluser]marlar[/eluser]
I second Aardvark. In fact that is the one I use the most. Fantastic for CSS debugging.

But also:

Web Developer
Lot of these bookmarklets: https://www.squarefree.com/bookmarklets/webdevel.html

[eluser]sandwormusmc[/eluser]
Definitely FireBug. Another one I've used in the past is "Remove cookie for site", which helps if you're debugging cookies and want to only blast the current page cookies instead of clearing your personal info every time.

http://www.dwipal.com/removecookiesextension.htm

[eluser]Michael Wales[/eluser]
I'm trying to recall from memory (I'm at work, which has significantly less extensions than at home):

Dev:
FireBug
YSlow
HTML Validator
CSS Viewer
FireFTP

Personal:
Download StatusBar
del.icio.us

[eluser]fredwu[/eluser]
Some more additions:

Console 2
JSView
View formatted source

And how come no one has mentioned IETab?

[eluser]Phil Sturgeon[/eluser]
Nobody has given a link for Web Developer, so there ya go! That thing is pretty damn awesome and does alot of things that multiple smaller extentions normally handle.

Another tastey one is Stealther. Handy if you want to ignore sessions and disable cache with a simple click. Also useful for hiding any "late night research" you happen to be doing.

[eluser]David Cassidy[/eluser]
A few others that were mysteriously not included:

Force-Content Type
Useful to those developers that understand that there is a difference between the incorrect use of the text/html mimetype and the correct use of the application/xhtml+xml mimetype and want to see how their XHTML pages render when the right mimetype is used. (More info: http://www.webdevout.net/articles/beware-of-xhtml )

Greasemonkey
Loads javascripts that can be useful for any number of situations - for instance, disabling custom styles in that blackhole we like to call MySpace and similar sites.

Mozilla Accessibility Extension
Hopefully you care about accessibility. If not, you should! (Visit http://www.w3.org/WAI/intro/accessibility.php for more information.)

User Agent Switcher
Various uses. Especially good for testing for those developers trying to offer alternate content for search engine robots.

If you have yet to download them and realize it yet, the two most important extensions mentioned in all of the replies thus far are the Web Developer's Toolbar and last - but certainly not least! - is Firebug.