Video playback on Vimeo.com is broken in the latest Nightly build (Built from https://hg.mozilla.org/mozilla-central/rev/c45f6e6d6005). You can hear the audio, but the video portion is just black. Tried with a clean profile, same behavior. Verified to not be a Vimeo issue as the videos playback on IE and Chrome. I could not see any errors in the browser console. Unable to replicate the issue on other sites like YouTube or DailyMotion.
Problem is caused when pref 'layers.offmainthreadcomposition.enabled = true'. False results in the video playing.
I was able to test this issue in the 7/5 Nightly build and the video played fine. I haven't tested 7/6 or 7/7 yet to see where the regression might be hiding.
setting pref media.windows-media-foundation.enabled = false also fixes the problem with OMTC enabled.
Might be a dup of https://bugzilla.mozilla.org/show_bug.cgi?id=1035101