How is it not properly supported at the moment? Genuinely curious, I don’t use PWAs a lot but when I have it has worked fine with Firefox.
Not sure, but Wiki has a bunch of footnotes that may explain
https://en.wikipedia.org/wiki/Progressive_web_app#Browser_support
Edit: it’s just this article, which seems like too much fluff to bother reading https://www.fastcompany.com/90597411/mozilla-firefox-no-ssb-pwa-support
This is not so much related to “proper” PWA support, but you can’t install PWAs as desktop apps from Firefox, whereas you can from Chrome/Edge. It’s the only reason I use Edge, just to locally install PWAs.
I use this addon to get “installable” PWAs and it works great for me. I use three installed PWAs regularly and have several more installed.