• Remove firefox snap

      sudo snap remove --purge firefox

  • Add PPA from mozilla team

      sudo add-apt-repository ppa:mozillateam/ppa

  • Ensure that the PPA is higher priority than the snap. Create the file /etc/apt/preferences.d/99mozillateampp with the following content

      Package: firefox*
  Pin: release o=LP-PPA-mozillateam
  Pin-Priority: 1001

  Package: firefox*
  Pin: release o=Ubuntu
  Pin-Priority: -1

The first stanza marks the mozillateam PPA release as a very high priority; the second
stanza marks the Ubuntu release as a very low priority.

  • Install the .deb version of firefox using apt

      sudo apt update
  sudo apt install firefox

Note: apt will warn that you are downgrading firefox. Ignore this, and go ahead with the 
installation. The warning happens because Ubuntu snap has a version number that includes 
an 'epoch' field, so that the snap version number is always greater than the .deb version number. 
This warning only happens when you first install the .deb version.