Oh, I've found out that this makes gtk3 gestures work, but not in firefox:
unset GDK_CORE_DEVICE_EVENTS
kwin --replace &
plasmashell --replace &
I don't know where and why GDK_CORE_DEVICE_EVENTS is set. Something related to erratic
scrolling behavior, but is known to break gtk xinput2.
( Though still no touch in firefox :( )