Step 1: Uninstall the Firefox Snap:

sudo snap remove firefox

Step 2: Create an APT keyring (if one doesn’t already exist):

sudo install -d -m 0755 /etc/apt/keyrings

Step 3: Import the Mozilla APT repo signing key (if wget is missing, install it first):

wget -q https://packages.mozilla.org/apt/repo-signing-key.gpg -O- | sudo tee /etc/apt/keyrings/packages.mozilla.org.asc > /dev/null

Step 4: Add the Mozilla signing key to your sources.list:

echo "deb [signed-by=/etc/apt/keyrings/packages.mozilla.org.asc] https://packages.mozilla.org/apt mozilla main" | sudo tee -a /etc/apt/sources.list.d/mozilla.list > /dev/null

Step 5: Set the Firefox package priority to ensure Mozilla’s DEB is always preferred. If you don’t do this the Ubuntu transition package will be reinstalled, and you’ll have the Firefox snap:

echo '
Package: *
Pin: origin packages.mozilla.org
Pin-Priority: 1000

Package: firefox*
Pin: release o=Ubuntu
Pin-Priority: -1' | sudo tee /etc/apt/preferences.d/mozilla

Step 6: Finally, use APT to remove Ubuntu’s pretend Firefox DEB (if still present) and installed the real-deal from Mozilla’s repository:

sudo apt update && sudo apt remove firefox

sudo apt install firefox

Step 7 (Optional): To use a localised version of Firefox (i.e., see the UI in a language other than American English) you need to install the corresponding language package. For example, to install French language support run:

sudo apt install firefox-l10n-fr

A list of all available language packs (listing the package name you need to install) can be seen by running the apt-cache search firefox-l10n command.

Once the repo has been set up and you’ve installed Firefox from it you can go ahead and open it. Open the app picker/menu on your Ubuntu (or flavour), click the Mozilla Firefox icon, and the non-Snap Firefox build, and you can continue on

Be sure to pin the icon to the Ubuntu Dock (or equivalent if you’re using an Ubuntu flavour) for easier access.