My main PC runs Ubuntu and in the past Apple have gone out of their way to make it difficult to watch the trailers on Linux so I just assumed they had deliberately broken something. I had a quick hunt around the web and couldn't see anyone else complaining that things had changed recently so I decided it had to be a problem on my machine. It took me a while to track down the problem but I thought it worth mentioning here just in case it trips up anyone else. Essentially VLC was the cause of the problem.
I use VLC as the main video player on my computer as it works well with DVDs and almost any file format/codec combination you throw at it. However, I'd recently had a problem with the audio and video going out of sync. While trying to fix that problem I had done a complete reinstall of VLC. As well as reinstalling the main application this had also reinstalled too Firefox plugins. It turns out that one of these plugins seems to interfere with the Totem based QuickTime plugin. I'm guessing that the VLC plugin takes presedence and then fails for some reason. The easy solution is to simply disable one of the VLC based Firefox plugins. You can do this from the addons page in Firefox (accessed by entering
about:addonsin the address bar). You should find two plugins named 'VLC Multimedia Plug-in'. The difference between them is that one states it is compatible with Totem. Leave this one alone and disable the other one and hey presto! QuickTime movies should start playing again.
Given that before the reinstall of VLC trailers had been playing properly, I'm guessing that I'd worked this problem out before and then forgotten all about it (memory like swiss cheese and all that), so hopefully this post should at least remind me of the solution next time I have the same problem even if it doesn't help anyone else.