I would try to clear both the cache and all cookies. If that does not help, I would check what headers this server is sending, because this sounds like the server is sending a bad MIME-type. Starting in Firefox 7 there are strict rules for headers that are send by servers. It is also possible that an add-on is causing problems, so you may want to try the Firefox Safe Mode as well.

There are also some sites that send bad pages because they do not recognize the useragent. In that case changing the user agent string may help
See for example https://support.mozilla.com/nl/questions/899115