Install non-snap Firefox

Remove the old snap-based Firefox:

sudo snap remove firefox

Enable access to the official Mozilla releases:

sudo add-apt-repository ppa:mozillateam/ppa

Prefer the non-snap version over the snap version:

echo '
Package: *
Pin: release o=LP-PPA-mozillateam
Pin-Priority: 1001
' | sudo tee /etc/apt/preferences.d/mozilla-firefox

Make sure automatic updates are enabled:

echo 'Unattended-Upgrade::Allowed-Origins:: "LP-PPA-mozillateam:${distro_codename}";' | sudo tee /etc/apt/apt.conf.d/51unattended-upgrades-firefox

Install non-snap firefox:

sudo apt install firefox