Firefox CSS issues

Firefox is having issues with my CSS. My site worked fine, but overnight the CSS seemed to dissapear from the site, and now it refuses to have any background besides white. I put a backed up version up, but I’d still like some help figuring out why this doesn’t seem to always work.

PS: the page as it was before seemed to not recognise any CSS at all in firefox, although IE and Safari recognised it right away. Is this just a firefox issue?