Greasemonkey, to automate a couple of tasks so I don't have to click as much on zilla and while testing stuff.

Firebug and web developer are a given.

Hmmm, and those are the ones I can't live without when it comes to dev; the other extensions I have aren't related, or are only tangentially related.

ETA:

The shell bookmarklet: https://www.squarefree.com/bookmarklets/webdevel.html

lets me run snippets of code quickly. (Firebug has its own console, but I like the bigger space and lighter feel that this provides, for a quick peek under the hood)