Possibly related: https://www.fxsitecompat.com/en-CA/docs/2016/flash-is-forced-windowless-mode-on-firefox-for-64-bit-windows-affecting-stage-3d/
Possibly related: https:/ /www.fxsitecomp at.com/ en-CA/docs/ 2016/flash- is-forced- windowless- mode-on- firefox- for-64- bit-windows- affecting- stage-3d/