Bug#921656: firefox ignores theme setting in .config/gtk-3.0/settings.ini

[sergio]

Package: firefox
Version: 65.0-1
Severity: normal

firefox  (same as thunderbird #921524) ignores

gtk-application-prefer-dark-theme = true

from

.config/gtk-3.0/settings.ini

but respects

$GTK_THEME=Adwaita:dark

All other gtk-3 apps works fine.