
Anyway, if you have this problem:
In Chrome go to Settings/Advanced/System/ and there switch off "Use hardware acceleration when available"
In Firefox go to Preferences/Performance, then if "Use recommended performance settings" is ticked, untick it and then untick "Use hardware acceleration when available"
Thanks Stakk!
p.s. After doing the change you need to restart Chrome or FireFox for the effect to take place.