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

[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.