Installation is a snap...
sudo curl -fsSLo /usr/share/keyrings/mullvad-keyring.asc https://repository.mullvad.net/deb/mullvad-keyring.asc
echo "deb [signed-by=/usr/share/keyrings/mullvad-keyring.asc arch=$( dpkg --print-architecture )] https://repository.mullvad.net/deb/stable $(lsb_release -cs) main" | sudo tee /etc/apt/sources.list.d/mullvad.list
sudo apt update
sudo apt install mullvad-browser
... for Linux users accustomed to adding new repositories and working with apt...
[Aside: tried to get my Chromebook running linux but could not get the firmware write-protect removed to allow me to flash the UEFI and install linux, so I only have command-line linux for now]