I was stuck on this problem for the majority of the day today and it was driving my crazy, i figured i would share my particular solution with everyone. Almost every place I looked up for an answer to this question had something to say about how the server is configured wrong, when i reality it had something to do with my master page!
If you cant figure out why your styles just wont show up in Firefox see if this is a fix for you too. Check the master file, make sure that you have your stylesheet linked correctly. If all is well there make sure you do NOT have a content placeholder in your header tags. For some reason if firefox can see that content placeholder in the head it will void the rest of the tags up there except for the title tag.
This just happened to be a fix for me and i hope it can help someone else!
