https://support.mozilla.org/en-US/kb/install-firefox-linux

Install Firefox .deb package for Debian-based distributions (Recommended) To install the .deb package through the APT repository, do the following:

Step 1: 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

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"}'

Step 2: add the Mozilla APT repository 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 3: Configure APT to prioritize packages from the Mozilla repository:

> echo '
Package: *
Pin: origin packages.mozilla.org
Pin-Priority: 1000
' | sudo tee /etc/apt/preferences.d/mozilla

Step 4: Update your package list, and install the Firefox .deb package:

> sudo apt-get update && sudo apt-get install firefox
> sudo apt-get install firefox-l10n-de