I've been experiencing this too! On only one of my machines. I did a complete Windows reinstall but the problem persisted.
It doesn't happen too often, but iirc I saw a text glitch sometime in the past few days. Moving my mouse over the glitched area or a small scroll solves it for me too.
Yeah I could get rid of it by scrolling or highlighting the text. I remember looking it up and only find like one small bug report a year ago. But I don't see it anymore. Are you on the newest FF? I don't use nightly and I haven't seen this problem in awhile.
Just checked, I'm on latest regular FF, non-nightly. I don't recall seeing it in the past couple days so maaaaybe it's resolved now? I'll keep an eye out for it in case I see it again.
It doesn't happen too often, but iirc I saw a text glitch sometime in the past few days. Moving my mouse over the glitched area or a small scroll solves it for me too.