Filed an upstream bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=705546
Sounds like a problem with recent changes to the text reformatting: http://dbaron.org/log/20111126-font-inflation
Version: unspecified
Severity: normal
URL: https://bugzilla.mozilla.org/show_bug.cgi?id=705546
This appears to have been resolved in recent days; something in the beta updates appears to have fixed it as a side effect. Mozilla people say no relevant changes on their end, and can confirm going back a few days in dailies they seem better now.
Not sure exactly what changed it, but possibly something affected the use of CSS to disable font adjustment or such. Yay!