Firebug 1.0 is a godsend for web developers. It's insanely good. It's still in beta but I've been using it for weeks and found it to be stable enough. Heck, even if it crashed the browser every five minutes (it doesn't), I would still use it. It's that good. Don't use the orange install button to install. Release Notes link just below it leads to the latest version. You'll have to use the orange install button now. My apology to Firebug guys for encouraging use of the other download link.
PS: I don't know why but installing firebug1.0-b7.xpi gives you 1.0b6 on Mac and 1.0b4 on Windows. scratch Restarting Firefox fixed this. My bad.
PPS: At least one person reported that Firefox 2.0.0.1 update is incompatible with Firebug. I haven't had any problem with the 2.0.0.1 update. If you encounter a problem, try switching back to the original Firefox theme.