I hadn't tried Firebug so I had to give it a day in court. The conclusion I drew was, yes, it's actually a pretty nifty add-on. I like that you can break it off and use it on a different monitor. It has code hinting which Edge didn't have (yet) but it's not quite as intuitive as the one in DW.
I would say that the only advantage of Edge is the fluidity of the real-time editing. With Firebug I had to right-click and select create new element or property when making new CSS rules, whereas Edge allows you to quickly add code just like writing it on a text document whilst still providing real time updates.
In conclusion, thank you for drawing my attention to Firebug. I will most likely use it, at least until Edge gets code hinting and multi-browser support.