I quote a previous response from FredMcD, which has been selected as "working"

- 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

Agreed. Also;

Separate Security Issue: Update your Flash Player Note: Windows users should download the active_x for Internet Explorer. and the plugin for Plugin-based browsers (like Firefox).

Note: Windows 8 and Windows 10 have built-in flash players and Adobe will cause a conflict. Install the plugin only. Not the active_x.

Flash Player Version: 24.0.0.186

https://get.adobe.com/flashplayer/ Direct link scans current system and browser Note: Other software is offered in the download. <Windows Only>

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>

See if there are updates for your graphics drivers https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration