This may be a long shot but it could help.
A while back when I was switching back and forth between KDE and Xfce, it occurred that suddenly my *firefox* fonts started looking awful.
After much digging around, it turned out that at some point a file ~/.fonts.conf was created, perhaps while playing with font settings in KDE. It had something to do with gtk, but don't ask me what.
Anyhow, after deleting that file, all fonts were restored and beautiful again (in Xfce that is).
So I guess my suggestion is: do you have such a file? From memory it has XML-looking config stuff in it. If so, try deleting/renaming it.
If not, well, sorry I couldn't be more useful...