Enshittification is mainly the result of a company having multiple different types of customers with opposing needs. Like Amazon has customers that buy things and businesses that sell things. YouTube has users who watch videos and advertisers that want to show as many ads as possible. Mozilla’s services are just sold to regular users so the risk of enshittification is minimal.
There’s stiff competition in the vpn market as well so they are incentivized to make the product good.