Yes, yet again, Apple discriminating. Not sure if by their own will, or just because they are stupid. Nevertheless, they got warned, and their action will clarify this point.
Now, why? Not sure why (there is a page with that story but I really never read it) but Apple has a trailers page. It is a nice page, with HD version of trailers. So, why not use it. The problem is that the HD versions, that are watched opening QuickTime (or supposedly, any other similar application), doesn’t work if you are using Firefox (on Mac OS X). You get a redirect to the original trailers index page.
Nevertheless, if you change your Firefox user agent string to the Safari user agent string, then the link will work properly. This means it is not a technology problem or anything. It is just Apple filtering what user agents can watch the movies.
So, if you still want to use Firefox and Apple trailers page for this task, then install, for instance, the Firefox user agent switcher plugin.
And a final thank you to João Neves for his precious help debugging this problem.