This is less a question than a note for reference, in hopes that if someone else searches for the same problem I just spent an hour and a half banging my head against, they'll find it. Someone else has no doubt documented it somewhere, but my Googling completely failed to help. Upshot is I had a nice, simple @font-face implementation working well. Then, along the line somewhere, I decided to move the font files to my media subdomain; as per some Yahoo recommendations, the media subdomain is at a different domain, so that cookies set for the main site content don't get fed along with all the requests to the media domain, since they're of no use there. Yes, it's a vanishingly small speed increase, but why not. Well, when I went to do final verification, all my @font-face rules had stopped working in Firefox (and Opera, I'm assuming for the same reason, though I haven't tested yet). Safari and IE both fine. Huh? A ridiculous number of experiments later and I finally figured out it has to do with the new implementation of HTTP access control. Upshot, as much as I can be bothered to read at 4:30am after an hour and a half of frustration, is that FF3.5 institutes some new HTTP header stuff wherein resources from a different domain will only be loaded if the HTTP headers specifically allow that particular cross-domain pairing. One of its intended effects, listed right at the top of the document, it to only allow font resources to be loaded from sites with explicit permission to do so. Which, being that it is not (so far as I've found) particularly widely documented, means that if you try to load a font from a domain other than the requesting one, it won't work, for no apparent reason whatsoever, and there are no errors reported in the FF console that I could find. Setting up the HTTP headers correctly is one workaround; the other is just sticking the fonts on the same domain. MAN that was frustrating. I'm sure there's a very legitimate and valid reason for doing this, but I suddenly hate FireFox so much more than I did yesterday. I'm sure it'll pass.