YouTube forcing mobile site on Mac Pro Safari 7.1.3

I don't know if it's to do with a recent update or something, but a couple weeks ago, I clicked on a link to a YouTube from Facebook, and it loaded the mobile version of the site, and when I clicked on the actual video to play it, it gave me the following error:

QuickTime Player can't open "video.3gp".

Now every time I go to YouTube, it forces the mobile version of the site and gives me that error.

N.B. If I load up Chrome, it works fine (of course), and I'm not running any extensions in Safari, everything is always set to default on the Mac.