This is how I downgraded firefox-esr to the latest 102.* package:
(assumes you still have it in your package cache):
sudo dpkg -i /var/cache/apt/archives/firefox-esr_102.15.0esr-1~deb11u1_arm64.deb
I then ran this to prevent it from being upgraded:
(originally posted by @pajuky, modified by me into a shell snippet pastable via ssh)
sudo tee /etc/apt/preferences.d/60_pin_firefox_esr.pref << 'EOF'
Package: firefox-esr
Pin: version 102.*
Pin-Priority: 1000
EOF
I’m going to hold onto 102.* until someone takes the time to update the PureOS package firefox-esr-mobile-config so it works with the latest firefox-esr.
Not sure if you’re referring to firefox-esr or some other package(s). However, the technique should be roughly the same for any Debian package.