There is a common issue with quicktime videostreams under linux. So this is a simple trick how you can make it working under the Ubuntu linux.
1) Uninstall all the video plugins you have got with your firefox installation (for example the vlc plugin)
2) Install the mplayer package and the quicktime extentions for it.
3) Install the mozilla-mplayer plugin
4) Reinstall firefox.
That's it. Go to apple.com/trailers, it should get working now