This is because the desktop browser uses the full "Gecko" renderer, but mobile uses "GeckoView", that doesn't have that implemented yet.
The actual thing is that simply each app embedding Gecko needs to be brought up to speed separately, and if Android is lagging behind, it just is, "GeckoView" or not. (Even before the invention of GeckoView, due to understaffing Android Firefox used to lag behind in terms of multi-process capability, so nothing new under the sun…)