Well, I don't know either what's the cause. I tried to delete cache, as well as cookies. Maybe some plugin is a cause. Hard to say. I believe that the clean re-install of FF would help but I don't want to do it just now because everything else is rendered - as far as I know - properly.
Maybe when I have more free time..
In terms of fonts, I have no problem. I use Tahoma as a system default without any font smoothing and it suits me. I have LCD Eizo L568 (I plan to replace it with 22" S-PVA Eizo S2231W by the way
- when I have some free money..)