Rated 5 out of 5 stars
Essential for JS developers for FF! Chromebug is also critical for Addon developers. Please update Chromebug for FF6+. Looking back, I don't know how I was able to develope addons without Chromebug before!
Firebug's slower but nicer/better thought out compared to webkit debuggers. The only major "flaw" is that it makes programmers lazier - I used to spend more time to carefully think about my JS code when a bug's found, now I just step through in Firebug and find/fix the problem quickly.