Just upgraded the wife’s Mac to Firefox 3, seemed to be painless and straightforward.  Until she went to use it the first time herself and some sites would show garbled text, what looked like squished fractions for certain text on web pages.  Only certain sites, only certain text.  Wikipedia is one that is usually affected.

Unfortunately not much information can be found on MozillaZine or most support forums.  Mainly because it’s kind of hard to describe or find the right search terms for.  But I did find a support post finally that pointed me to the true fix: disable the Helvetica Fractions font in Font Book.

Hope this helps save an hour of someone else’s time 😉