URL: http://trailers.apple.com/
Browser / Version: Firefox 42.0
Operating System: Mac OS X 10.10
Problem type: Video doesn't play

Steps to Reproduce
trailers.apple.com requires the QuickTime plugin for Firefox but serves HTML5 video (M4V) to Chrome.

1. On a Windows machine without Apple's QuickTime plugin installed, navigate to: http://trailers.apple.com/
2. Try to watch a movie trailer, such as http://trailers.apple.com/trailers/disney/thegooddinosaur/

Expected Behavior:
The video should play.

Actual Behavior:
The play will display a blank gray rectangle instead of the video or an error message saying "A plugin is required to display this content." But if you fake Chrome's User-Agent and reload the movie trailer page, then the video will play correctly.

3. Install the "User Agent Switcher" add-on: https://addons.mozilla.org/en-US/firefox/addon/user-agent-switcher/ There are multiple User-Agent switching add-ons, but it is important to use this one because it changes both Firefox's User-Agent header and navigator.userAgent property. Apple's trailer site checks the navigator.userAgent property.
4. Switch your User-Agent string from Firefox to Chrome, e.g. "Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36"
5. Reload the same movie trailer page.

RESULT:
The video now plays in Firefox even though the QuickTime plugin is not installed.