Well, after causing the distress to many, I guess the story worked. Sure was an “unexpected” and an “uncalled for” event. Luckily it was “fixed” soon enough to maintain some of the Mozilla Firefox enthusiasts…actually of which I must be one…
Every day and sometimes multiple times during the day, I have to re-install my theme as it disappears and goes to plain black. Im using 68.0.1 (64-bit) version, which is apparently up to date.
This could indicate an incompatibility with your graphics card/chipset driver software. You can test turning off hardware acceleration in Firefox, and look for any driver updates.