hello, please try updating your graphics driver, or in case this doesn't solve the issue or there is no new version available at the moment, disable hardware acceleration in firefox > options > advanced > general.
Try to set the gfx.content.azure.enabled pref to false or if this didn't help disable Direct2D by setting the gfx.direct2d.disabled pref to true on the about:config page and leave hardware acceleration otherwise enabled.
Thanks for the quick answers.
First I updated the graphics driver, but nothing changed. If I disable the hardware acceleration the fonts change a bit but the problem seems to be solved.
As for the second solution: the first pref change made all the right click popups completely black, the 2nd pref did kinda the same as with the disabled hardware acceleration.
All in all the glitch seems to be repaired, but only due to the decrease in quality (I guess).
The difference can be seen in the pic.
Once I highlight the glitched area, it goes away, but as soon as I scroll it appears somewhere again.
Thanks a bunch for the help. If there is something more to try, please do not hesitate to help