In the setup I was using (Mac Firefox 22 with Theme Font & Size Changer, plus the previous version of Firebug), the problem absolutely did involve the interaction of those two extensions. I wasn't just guessing: I spent a lot of time testing, disabling different groups of extensions, and relaunched repeatedly to ensure I was right -- before I posted in this forum.
I've now updated Mac Firefox to version 23.0.1, which allowed me to update Firebug to the current version as well, and the problem seems to be basically resolved. However, the way Firebug shows me that one of those buttons is "pressed" is so subtle (a slight darkening/shadow), especially on my crummy auxiliary monitor where I normally keep the Firebug window, that it's hard to see whether it was pressed or not. There's also something strange: in the previous setup, if I disabled Theme Font & Size Changer, Firebug sometimes showed the pressed state with the darkening/shadow effect, or sometimes by turning the label white -- for the same buttons!
Interestingly, the appearance of the pressed state in Windows 7 Firebug is completely different (a kind of outline box) and *much* clearer than in the Mac version. But then, Windows Firefox (and its extensions) has always worked better than the Mac version in numerous small ways.
If anybody knows of a CSS selector for the pressed state of those Firebug buttons (Clear / Persist / Profile / etc.) that I can tweak in userChrome.css -- my original request -- I'd still like to know.