The VP9 estimizer will force-enable webm/VP9 on fast machines, even if media.mediasource.webm.enabled = false. How does one disable webm/VP9 then? The VP9 estimizer logic should be reversed: it should veto media.mediasource.webm.enabled = true on slow machines instead of forcing VP9 even when media.mediasource.webm.enabled = false. FF 48 is affected because bug 1230265 landed in 48. Whether 47 is affected depends on whether bug 1230265 is uplifted to 47.

I'm not sure that approach is wise either media.mediasource.webm.enabled is false by default on all platforms but linux If the user sets it to true, shouldn't it be active regarless of the speed test. Surely we can't tell the user: "sorry, we're not letting you do what you want because we think your machine is too crap" To disable the VP9 estimiSer (it was written in NZ, so her Majesty's spelling is preferred) one case simply up media.benchmark.vp9.threshold from 150 to say 10000. The issue is the same anyway for people with no h264 decoder or no hardware acceleration: they can't disable VP9 there either. So should we have a pref to simply disable the media.mediasource.webm.enabled override ?

Review commit: https://reviewboard.mozilla.org/r/42889/diff/#index_header See other reviews: https://reviewboard.mozilla.org/r/42889/

Comment on attachment 8735653 [details] MozReview Request: Bug 1260305: [MSE] Add pref to disable webm override. r?kentuckyfriedtakahe https://reviewboard.mozilla.org/r/42889/#review39701 Lets not add more prefs here.

SGTM