Recently I ran into an issue with FireFox where it indicated that our CSS file was of the wrong MIME type. The error read:
Our first thought was the MIME type in IIS on this server must not be setup correctly. So we checked that out and it was set correctly. It turns out that the issue was with our DOCTYPE.
The DOCTYPE was:
But by changing the DOCTYPE to transitional, we were able to get it to load and function correctly.
It worked for us. But it is worth noting that even after this change, FireFox and Firebug still displayed a warning about the MIME type.