FYI: On install, Firefox 5 said it was not compatible with Firebug 1.7.3 and disabled it. I happened to be sitting next to 5 or 6 Mozilla devs at the moment (open source bridge conf) and one of them told me to install Firebug fresh from the site. I downloaded and installed the 1.7.3 xpi and it worked without complaint. No idea what's up with that.