Can you check under Help --> About and see if Firefox updates to version 56.0.1 please.

Can you provide a list of your extensions as no information came through about them. Nor info re Video Card.

Can you please expand on does not work .

Please note that Windows 10 can and will change your Hardware Drivers to their own. This can cause Issues. https://www.howtogeek.com/302595/how-to-stop-windows-10-from-automatically-updating-hardware-drivers/

https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

https://support.mozilla.org/en-US/kb/html5-audio-and-video-firefox

If you have a outdated Flash or Flash 27 please use Adobe Flash Uninstaller https://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html then get a new version https://get.adobe.com/flashplayer/otherversions/ Step 1: Select Operating System Step 2: Select A Version (Firefox, Win IE . . . .) Note: Other software is offered in the download. <Windows Only>

Please let us know if this solved your issue or if need further assistance.

I just saw the no-reply in your email. So my replys are on their way to Nunki (Sigma Sagittarii). Not working means it shows the 1st frame and there's an expanding gray circle behind the start triangle/arrow but no motion, same in safe mode. Help about yields no update. It works in opera but they're chinese. And it works in firefox in Ubuntu 16.04 in Virtualbox on the same Win 10 System Model HP Pavilion x360 Convertible HP Pavilion x360 - 15-bk193ms Processor Intel(R) Core(TM) i5-7200U 2.50GHz, 2712 Mhz, 2 Core(s), 4 Logical Processor(s) The embeded youtube video at https://www.engadget.com/2017/10/10/intel-superconducting-test-chip-quantum-computing/ works. But the same video at https://www.youtube.com/watch?v=qEvrPYmvBIY does not ??? I ran Ninite.exe today and it updated firefox to 56.01 and ytube is still motionless.

You may want to take your issue here : https://askubuntu.com/ to see if other users are experiencing this issue and if it is a Ubuntu issue.

Please let us know if this solved your issue or if need further assistance.

it works in Ubuntu.

the problem is youtube in firefox in win 10 only . fails in safe mode too

Do you get the "static" background with a specific error message, or... what happens exactly?

I see the 1st frame of the video like normal. The button turns from black to red ( with white arrow ) when the mouse pointer is in the button. When i click it a transparent gray circle expands from behind the button but nothing happens, no motion, no messages, nada.

Another question, why the unreadable butttons?????

fredDag said

I see the 1st frame of the video like normal. The button turns from black to red ( with white arrow ) when the mouse pointer is in the button. When i click it a transparent gray circle expands from behind the button but nothing happens, no motion, no messages, nada.

Your YouTube player sounds quite different from mine. For me, videos autoplay when I open the page, and the control bar has white icons on a dark background. (Example attached.) If you have some other arrangement, perhaps some files are being blocked? If you have the identical issue in Firefox's Safe Mode, then it's not an extension. Perhaps it's an external filter (security software, parental control, proxy, etc.)?

Another question, why the unreadable butttons?????

White text on a light colored background for the active button is not good. I think on Windows 10 Firefox takes its theming cues from your Windows theme and/or settings at the OS level. Off the top of my head, I'm not sure which setting that might be (Windows 10 may not have the same access to set colors as earlier versions.)