It seems that the QuickTime plugin in Snow Leopard (
/Library/Internet Plugins/QuickTime Plugin.plugin) cannot be forced to open movies (eg. the ones at trailers.apple.com) in the old player, "QuickTime Player 7" (which is an optional component and once installed, it's at /Applications/Utilities/QuickTime Player 7). If you don't know why this is necessary, then you didn't try to open a QuickTime movie (*.mov) from a website yet. But all is not lost yet, because there's a workaround: exit QuickTime X and start QuickTime Player 7 _before_ you'd open the movie in your browser. It appears that in case there's already a running instance of a QuickTime player, the browser plugin will use that one (if there's no running player instance, then it'll always start QuickTime X).
