2. It seems, that LMDE 2 needs (in my case) additional tweaking of the scrolling behavior in Firefox (although I use the smoothwheel extension): Go to about:config and change the layout.frame_rate to 480 (in my case -> I have a 120 hz Screen) and the layout.frame_rate.precise to true.
3. To get a smoth scrolling in Firefox, as like known from your smartphone, where no stutter occurs, just edit these 2 about:config settings to true: layers.offmainthreadcomposition.enabled and layers.offmainthreadcomposition.async-animations
That's all... maybe I find out something more in the future... I will post it!
Does anyone find out similar things you won't find on the internet elsewhere?

My Specs:
LMDE 2 Cinnamon 64 bit
Nvidia 9600 GT
Iceweasel 38.04
Cu
Dr. Octagon