While firebug is clearly superior, web inspector is much more stable. It ends up being a combination of both (or one of the older, less-featured, more-stable versions of firebug).
I don't use Firefox at all, because for what I do it's clearly inferior. I don't think I've launched Firebug to do anything other than log in the last 6 months.
Firebug to log in?
He's using Firebug to log javascript errors via console.log, to test his stuff in Gecko / Firefox