Well, I had the same problem.. I downloaded Google chrome, as I figured it was faster and used less resources. It works fine in google chrome, but not in firefox. If I have firefox open while I play MTS with chrome, it crashes. It only works in Chrome, (for me), but then it works fine!
I think I saw something about the new way MTS plays is "flash", or something like that. No clue what that is, I'm not very technically savvy, lol!

It might be something else though, maybe your drivers or something, I dunno.. Try Chrome first!