Maybe Canonical has changed something for the Firefox Snap Package. I have added to the Mozilla Perferences File for prioritizing the Mozilla Source a negative Priority for the Ubuntu Package so that the File looks loke that:
Package: *
Pin: origin packages.mozilla.org
Pin-Priority: 1000
Package: firefox*
Pin: release o=Ubuntu
Pin-Priority: -10
Because I don't use Snap, I uninstalled it and added a nosnap File, too to downgrade it with the Content:
# To prevent repository packages from triggering the installation of snap,
# this file forbids snapd from being installed by APT.
Package: snapd
Pin: release a=*
Pin-Priority: -10