Basically what happens is, that Firefox doesn't load my typefaces when my index.html is not in the root folder.
If I put the same index.html in a sub folder (and relink the href's), everything loads fine but the typefaces. If I upload them to my server, everything works fine. So the problem is just locally.
It's really weird.
This is what I use for my font-face rule:
@font-face {
font-family: pt_sans_condensed;
src: url(../fonts/PTN57F.ttf);
}
(I only have this in firefox)