Create a directory to store APT repository keys if it doesn't exist:

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

Import the Mozilla APT repository signing key:

wget -q https://packages.mozilla.org/apt/repo-signing-key.gpg -O- | sudo tee /etc/apt/keyrings/packages.mozilla.org.asc > /dev/null
If you do not have wget installed, you can install it with: sudo apt-get install wget

The fingerprint should be 35BAA0B33E9EB396F59CA838C0BA5CE6DC6315A3. You may check it with the following command:

gpg -n -q --import --import-options import-show /etc/apt/keyrings/packages.mozilla.org.asc | awk '/pub/{getline; gsub(/^ +| +$/,""); if($0 == "35BAA0B33E9EB396F59CA838C0BA5CE6DC6315A3") print "\nThe key fingerprint matches ("$0").\n"; else print "\nVerification failed: the fingerprint ("$0") does not match the expected one.\n"}'

Next, add the Mozilla APT repository to your sources.list:

    For Debian Bookworm/Ubuntu Noble and Older: 

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

    For Debian Trixie/Ubuntu Resolute and Newer 

sudo tee /etc/apt/sources.list.d/mozilla.sources > /dev/null << EOF
Types: deb
URIs: https://packages.mozilla.org/apt
Suites: mozilla
Components: main
Signed-By: /etc/apt/keyrings/packages.mozilla.org.asc
EOF

Configure APT to prioritize packages from the Mozilla repository:

sudo tee /etc/apt/preferences.d/mozilla > /dev/null << EOF
Package: *
Pin: origin packages.mozilla.org
Pin-Priority: 1000
EOF

Update your package list, and install firefox (or one of firefox-esr, -beta, -nightly, -devedition):

sudo apt-get update
sudo apt-get install firefox