Attempting to open ANY video on youtube's mobile site brings up "this link needs to be opened with an application" with no way I can see of actually playing the content. This is true even for videos which I can play fine on the desktop site (still accessing via mobile browser). I'm using FF19 on a gingerbread galaxy ace 2 (rooted, FWIW), no flash support, stagefright.force.enabled is set to true.
so there's no way to force firefox to try to play the video on the mobile site, even though it can manage to play the same video without issue when delivered through the desktop site, and when other browsers, also without flash support, can play the video without issue?
When on m.youtube.com the site provides links that launch in the YouTube app to be installed. In most cases this provides a better playback experience. Unfortunately there is no fallback mode designed by m.youtube.com when YouTube app is not present.