Some time ago, YouTube had switched to HTML5 videos for all modern browsers including Firefox. After doing some tweaks you can play all HTML5 videos in Firefox even without the Flash plugin installed. In some cases, e.g. when YouTube detects that something is wrong with your browser, it might switch automatically to using Flash for videos instead of HTML5, but there is no way to control this yourself. Let me show you a way to switch between HTML5 and Flash manually.
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