For some reason, your “downgrade” is showing (100.0.1-1 => 100.0-0.1) , but 100.0-0.1 is incorrect!
Try:-
sudo pacman -U /var/cache/pacman/pkg/firefox-100.0-1-x86_64.pkg.tar.zst
For some reason, your “downgrade” is showing (100.0.1-1 => 100.0-0.1) , but 100.0-0.1 is incorrect!
Try:-
sudo pacman -U /var/cache/pacman/pkg/firefox-100.0-1-x86_64.pkg.tar.zst