Created attachment 875336 [details]
Screenshot: Added the Norwegian keyboard layout to the GNOME settings
Obviously GNOME uses its own keyboard settings that only affect GNOME
applications (which include Firefox) by default, and they override the
system-wide settings.
KDE behaves in a similar way, but IIRC they default to the system-wide keyboard
settings, not just always US. But this may be a bug in this specific GNOME
version.