Isn't being installable as an app kind of the most baseline feature of PWAs? To my knowledge desktop Firefox doesn't let you do that without an addon like this.
IIRC Firefox did use to support installing of PWAs until about a year ago. It still boggles my mind why they dropped it.
Bizarre decisions about features is run of the mill these days for Mozilla. I had to use FF a few days ago (WebKit derivatives can't load massive GitHub pull requests) and I truly missed the browser. But I use PWAs, so I have to use Chrome.
I'm not sure why you find Chrome necessary for PWAs, I use Discord, Spotify, Zoom, etc. all exclusively from within Firefox.
That's not what a PWA is, a PWA is installable, and Firefox doesn't have that.