Quick update for my... um... 7 readers? I lost track. Can't count upwards from 5.
Anywho, I tried the recently launched YSlow extension and it's quite solid. No gripes yet, but I pretty much just pressed all buttons without actually using it for realz.
It's by far the best firebug extension I've seen yet, very complete and loaded with nice stuff. Doesn't work with IE yet, but then again nothing does >:)