Firefox 64.0 renders content transparent

Having the same problem, and can't find enough info about it online!

Strange "ghost images" / transparency effect in full-screen videos

For me, setting layers.acceleration.force-enabled to true in about:config fixed the issue, but I took a big hit in performance and FPS of my videos. There was visible stuttering.

In another answer in the Mozilla Support Forum, one other solution that was suggested and indeed fixed the issue was changing gfx.xrender.enabled to true, instead of forcing hardware acceleration.

I don't know which of the two is better.

Found a solution, but I'm not sure how this became an issue in the first place.

Quite generally looking for graphics problems in Firefox I stumbled upon this answer. I was looking in about:config for the value of the option layers.acceleration.force-enabled and found it manually set to true. After resetting it to the default value of false and restarting Firefox, the issue is completely gone.

So for my situation the solution was the opposite value to the one suggested by the other answer.