Just wanted to follow up this comment by saying that I've been playing with the latest Firefox nightly this morning and it's much more inline with what I want out of a browser's inbuilt dev environment. I'm really impressed.

Two things that still stand out as lacking when compared to Chrome are the JS console output and the Net panel output (really hate those popup windows).

But these things can be ironed out. Well done for getting Firefox moving in the right direction.