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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)