Hmm...I thought FF did some automagic on YouTube links. Oh well. - chrisofspades #FForever.

At some point, it got a bit funky - if you're able to get to a non-HTTPS version of the video, then it will auto-display (or if you have a specially crafted bookmarklet.) - Jennifer Dittrich

Ah non-https did the trick. Thanks Jennifer! - chrisofspades #FForever.