Under the SNAP version of Firefox, hardware support for multimedia codecs cannot be installed (or rather, I have not found a way to do so). Therefore, here is a guide to replace the SNAP package with a DEB package.
Please remember to save your bookmarks and settings !!!
This guide was taken directly from the Mozilla website, please go there if you are looking for explanations of the installation steps.
sudo install -d -m 0755 /etc/apt/keyrings
wget -q https://packages.mozilla.org/apt/repo-signing-key.gpg -O- | sudo tee /etc/apt/keyrings/packages.mozilla.org.asc > /dev/null
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
echo '
Package: *
Pin: origin packages.mozilla.org
Pin-Priority: 1000
' | sudo tee /etc/apt/preferences.d/mozilla
sudo apt update
sudo apt install -y firefox
sudo apt install -y firefox-l10n-de \
sudo snap remove firefox