Id like to get an answer for this too. I am using waterfox 56.2.0, could you confirm this one is good for you too?
I dont know about dithering, it still seems to me noone has proved it properly. But I noticed in new firefox it doesnt hurt when I look at photos, but reading does, so I suspect its font aliasing/shimmer. It should be possible to override browser changing graphics mode in preferences, but it does no change for me.
It is also possible its a conflict of old software (win 7) with browsers made for hi res modern screens.
I checked the difference between waterfox thats fine and firefox that isnt with magnifying glass, I cannot see anything so its probably more temporal and need to be recorded. Or its not magnified enough?