Sounds like a potential driver related suspend/resume issue, where we probably need to do a device reset, but we weren't told to.
Bug 1768885 Comment 2 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Sounds like a potential driver related suspend/resume issue, where we probably need to do a device reset, but we weren't told to. Does killing the GPU process resolve the issue without restarting Firefox?