The mobile Firefox needs the Flash Plugin to run the Flash version of videos on Youtube.

It could be due to Amazon Silk browser running some things remotely on the Amazon EC2 servers

Thanks for the reply. Is there a Flash Plugin for Firefox on Android? I think that I tried to find one when I first got my Kindle Fire HD and found out that there wasn't one. Somebody at Adobe needs to fill that gap. Silk must be doing something like you mentioned.

There is the old insecure Flash 11.1 Plugin that Adobe (and Google Play) has not supported since Android 4.0. As long as you have Android 4.3 or older then you can get the Flash plugin to work in manual install though Android 4.4 has made it so Flash will not work.

However Mozilla has made a workaround for Flash can still work in Firefox on Android 4.4 in as early as Firefox 27.0 if I recall.

There is this thread on getting Flash installed however the poster mentions Dolphin and not Firefox instead. http://www.amazon.com/forum/kindle?_encoding=UTF8&cdForum=Fx1D7SY3BVSESG&cdThread=TxTR71JR4GVXQM

Not that some sites (News sites are among some I encountered) have since lately been requiring users using Flash 11.1 on Android, PlayBook and Flash 11.2.x on Linux to upgrade to more current (not possible) to view their Flash videos.

Thanks for the info. It looks like I'm in over my head trying to get Flash Player videos to run on Firefox for Android. I'll just have to stick with Silk to play videos on my Kindle Fire HD.