FilkoSE -
Did you use negative numbers for the font heights? This is a known issue (not just with FF, as I understand it) and the solution is to be sure a minus sign precedes the number when setting font height in Fonts for Classic Widgets. I've found that using a negative number for the Titlebar text is useful in some cases as well. I can't begin to understand or explain why this is so, but it is. This info was included in Adam Najmanowicz's article "Ten things to check before uploading your skin" (or words to that effect) - should be able to find it in the WindowBlinds section here (maybe in OS Customization).
Using FF 1.0.4 on Win2k & XP with no font issues here (using negative numbers).
Cheers,
Daiwa