No, no, it's not the html file we are interested in. It's the HTTP headers sent by the server. If those doesn't serve it as 'text/css', moz doesn't even try to interpret it. (And it shouldn't, either, according to W3C.)
Go to <http://webtools.mozilla.org/web-sniffer/
> and enter the addy of your css file there, and check the Content-Type header.