It seems i finally fixed the issue. I installed this gnome extension and enabled it on firefox in gnome extensions:
Please add this solution to Zorin 17 per default or as option in settings gui. This would be awesome. Seems there are no more fullscreen issues even with fractional scaling enabled on 4K and 1080p multi monitor setup.
This Guy has the exact same multi monitor setup as mine and he has uploaded a gif which is showing the problems. These are exact as mine:
GIF:
