Unfortunately, the new web developers tools* built into Firefox represent a small subset of what Firebug and Webkit Inspector can do. Since Firebug is not compatible with Nightlies (Firefox 8) and Aurora (Firefox 7), it's a major regression.
Given that Nightly is the least stable kind of build and could have other problems along the way, we've reached a pretty good point with Firebug's compatibility.
Of course, you also have the Web Console if you're having trouble with Firebug (and I'll grant that this alone is not a replacement for Firebug!)
ObDisclaimer: I'm the product manager for devtools at Mozilla.
The web console is not a replacement for firebug. its just decent built in tools.
Firebug - guess what - supports Firefox 7 and probably 8 too. And of course, Firefox 5 as well which is the latest release of Firefox btw.
It's tiring to read the large amount of FUD against Firefox lately. People like you are the major regression to the interwebs. Very seriously.
Today I needed to determine if any resources were 404ing on a page. Yesterday I wanted to see the exact POST request my browser was sending. I did both on FF using the inbuilt developer tools. I'm sure they may not replace every piece of Firebug functionality, but I didn't say they did.
> People like you are the major regression to the interwebs. Very seriously.
I've flagged your post. Please read http://ycombinator.com/newsguidelines.html