New Update; Flash Player Version 19.0.0.226
https://www.adobe.com/products/flashplayer/distribution3.html Note: Windows users must download the “Internet Explorer” and “Plugin-based browsers” installers.

Note: Edge on Win10 does not use ActiveX version but its own Flash.

check this:

• see if there are updates for your graphics drive drivers

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

• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)

https://forums.adobe.com/message/4468493#TemporaryWorkaround

• disable hardware acceleration in the Flash plugin

https://forums.adobe.com/thread/891337 See also:

• http://kb.mozillazine.org/Flash#Troubleshooting

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

Did you disable autoplay?

You can check the media.autoplay.enabled preference on the about:config page. When you disable autoplay this way then the HTML5 media player might show the wrong state and you need to click twice to start the playing. You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

Is it this:

You load a page on YouTube, the player shows a pause button, and you can see it buffering, but nothing plays. Then if you click the pause button (it changes to a play button) and click the play button it plays normally.

In that case, cor-el's theory is the most likely explanation.

Before Firefox 41, scripts would send a Play command to the HTML5 media player to start the video to work around users who disabled autoplay. Firefox 41 stops the playback, but it does not fix the player controls, so you see a pause button even though the playback is halted. Hopefully there's a way to fix it in a future update.