I don’t agree with this characterization or comparison. Mullvad Browser–like Tor Browser–has a very specific use case it is intended for, it serves that purpose well but it is not intended to be used the way that the vast majority of people use their web browser.
For this reason, in my eyes it is not the right browser to be comparing to Librewolf which is designed for a different and broader scope of use. If you want to argue Librewolf shouldn’t be listed (an argument that I am sympathetic to, but not 100% onboard with), I think you should be comparing it to PG’s other recommendations (Firefox + Arkenfox, or Brave) which have a much more similar scope of use to librewolf than Mullvad Browser does in my opinion.
*For users of a certain proprietary operating system.
If you use any of the OSes recommended by privacy guides (or any Linux distro for that matter) it receives automatic updates like any other flatpak does.