Someone kindly told me that I had a stale firebug! Go here for the latest.
I recently managed to get an alpha firebug (1.2.0a7x) working well on FF3 by also installing the Web Developer add-on and restarting. I've confirmed that this workaround also helped a colleague (using a more recent 1.2.0a22x of firebug). The workaround doesn't appear necessary for 1.2.0a23x. In other words, this post is obsolete. Sorry for the interruption.
Thanks for reading.