[purple_ducks]

Author's using Windows..

Latest FF on Windows seems to have issues with rendering.

Loading up posts on old.reddit.com leads to the DOM glitching after a few seconds and elements disappearing.

Happens frequently on pages with lots of comments. Something glitchy going on.

[AstralStorm]

Your graphics driver is likely faulty, as I have not noticed any such issue with AMD Radeon Vega 64.

Try toggling GPU acceleration off in settings.