I'd like some recommendations for tools people use when developing
Specifically I'd like to know what browser add ons to use - but if there's anything else that is amazingly useful as a tool, I'd like to know!

I've used Chromes developer tools - awesome!
Chris Pederick's web developer plugin for Firefox - awesome!

Firebug?? I've seen this mentioned in loads of places. How is it different to Web Developer? Is it better or does different things?

What else would you recommend?