Dam. The original problem is back. Firefox is driving me nuts.

https://www.youtube.com/watch?v=3pN9WU1S8nQ Your browser does not currently recognize any of the video formats available. Click here to visit our frequently asked questions about HTML5 video

I use to run AdBlocker (or something like that) but see that it is not showing up in Add-ons, extensions, Plugins.

Frustrating.

This morning I see that when I watch an MP4, Firefox downloads the video. After download, it ask me What should Firefox do with this file? Default Windows Media Player.

Hello   G_Eak,

Would you try this please :

Type in the address bar   about:config   and press Enter.
(ignore the warning)

Type in the search bar and look for the preference :

media.play-stand-alone   and make sure its value is set to   true.

If you had to change the value, then close and restart Firefox.

Any luck ?

Thanks for the reply McCoy,

I had checked media.play-stand-alone. It was set to true. I gave up. After copying passwords & favorites file, I uninstalled Firefox, deleted profile and reinstalled Firefox. That fixed it. So far, all Youtube is working and videos play in Firefox.

I suspect there was an add-on pseudo running but not showing up in "Add-ons".

Appreciate the reply!!! issue closed.