Google's HTML 5 media player uses Media Source Extensions (MSE) now [1]. This enables them to use adaptive streaming implemented in JavaScript using DASH JS libraries.
Unfortunately MSE support is not complete in Firefox so the MSE using HTML5 YouTube player doesn't work. This will be why Google would be serving Flash. I don't know if they continue to serve the non-MSE HTML 5 player.
MSE support in Firefox is in progress. Nightly builds can enable it by changing the 'media.mediasource.enabled' about:config setting.
Don't do this though because it will definitely degrade your YouTube playing. Due to an incomplete feature it waits for the entire video to download before playing. There's a patch for this that gets playback working but there are still issues with the adaptive streaming itself (because the implementation is incomplete).
Follow bug 881512 [2] if you want to track support in Firefox for the "Get YouTube working with MSE" issue.
I wrote a bit about this [3] a while back and will do an updated post, if someone else doesn't first, once YouTube support is stable.
MSE is actually really cool and makes streaming media over HTTP much more pleasant. I'm not surprised they want to keep the number of codepaths they have to support low. As the GP mentioned, MSE is coming to Firefox soon anyway.
ActiveX is actually really cool and makes [whatever new feature] much more pleasant. I'm not surprised they want to keep the number of codepaths they have to support low. Have we been here before?
Hardly. The "new code/technology" is completely open spec/proposal and its implementation is open source in Chromium. In terms of "Google pushing it", the spec was co-edited by people from Microsoft and Netflix. In terms of "supported only by Chrome", this is strictly Mozilla's fault as there is nothing stopping them from implementing the spec today.
This article and a lot of comments here are really sensationalizing reality. Google needs to serve tons of video data efficiently so they are motivated to push things like MSE. Reading this as "Google crippling Firefox" seems really dramatic and contrived.
Unfortunately MSE support is not complete in Firefox so the MSE using HTML5 YouTube player doesn't work. This will be why Google would be serving Flash. I don't know if they continue to serve the non-MSE HTML 5 player.
MSE support in Firefox is in progress. Nightly builds can enable it by changing the 'media.mediasource.enabled' about:config setting.
Don't do this though because it will definitely degrade your YouTube playing. Due to an incomplete feature it waits for the entire video to download before playing. There's a patch for this that gets playback working but there are still issues with the adaptive streaming itself (because the implementation is incomplete).
Follow bug 881512 [2] if you want to track support in Firefox for the "Get YouTube working with MSE" issue.
I wrote a bit about this [3] a while back and will do an updated post, if someone else doesn't first, once YouTube support is stable.
[1] http://www.w3.org/TR/media-source [2] https://bugzilla.mozilla.org/show_bug.cgi?id=881512 [3] http://bluishcoder.co.nz/2013/08/20/progress-towards-media-s...