Thanks for the help!
SOLVED:
for some reason the “Neutral” cursor theme does not scale.
neofetch gives me
Icons: Papirus-Dark [GTK2/3]
according to this (under 2.2 LXAppearance) Firefox (and other apps) respects the cursor theme set in ~/.config/gtk-3.0/settings.ini
I changed gtk-icon-theme-name=Neutral
to gtk-icon-theme-name=Papirus-Dark
and it worked.
Shout-out to Blugil on Reddit (see bottom of post for solution)