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

Any video via Firefox are so slow sometimes stuck although any video is fine with other browsers. I think there is a problem in Firefox browser. All your suggestions did not work.

What media player is used if you check this via the right-click context menu of the player area? Is that an HTML5 media player or a Flash based player?

A Flash player shows a Flash context menu.