Mozilla does not want to support PWAs in Firefox, yet MDN has a PWA manifest and can be installed in Chromium.
(discuss.tchncs.de)
Nice, I was trying to figure out how to get that working with Firefox. But, to be fair, it's not Firefox that's supporting PWA, it's the mint webapp-manager which is only included with Mint and requires extra steps to install on other OSes. Not as straight forward as PWA being directly supported by Firefox.