There's also Firephp for getting the server to send additional trace data: http://www.firephp.org/

EDIT:

Also, this article has a good overview of multiple Firebug extensions: http://www.softwareishard.com/blog/firebug/list-of-firebug-e...

I know some people who love PixelPerfect for transferring Photoshop mockups to CSS.

Also one of the comments mentioned widerbug which I might try now that I have this weird 18.4" laptop: http://www.command-tab.com/2008/01/19/widerbug-widescreen-fi...