At the moment you can go back with previous version using pacman cache
$ cd /var/cache/pacman/pkg/
$ ls | grep firefox
firefox-81.0-1-x86_64.pkg.tar.zst
firefox-81.0.2-1-x86_64.pkg.tar.zst
firefox-82.0-1-x86_64.pkg.tar.zst
firefox-i18n-es-ar-81.0-1-x86_64.pkg.tar.zst
firefox-i18n-es-ar-81.0.2-1-x86_64.pkg.tar.zst
firefox-i18n-es-ar-82.0-1-x86_64.pkg.tar.zst
$ sudo pacman -U firefox-81.0.2-1-x86_64.pkg.tar.zst firefox-i18n-es-ar-81.0.2-1-x86_64.pkg.tar.zst
DX2300
you can see the error when launching from konsole