I love the Web Developer extension. I use it all the time in development.
Live HTTP Headers is sweet.
Venkman, the javascript debugger is HOT.
yeah, I used to use mozilla just for Venkman. its one of the reasons I downloaded FireFox. I haven't done any hardcore javascript at this job yet, but it sure comes in handy if you do some.
Yeah that developer extension is very helpful. Page resizing to test for resolutions without actually changing the res saves a headache.