I haven't personally used it but a lot of people say the PWA for Firefox extension works pretty well. You do need to install a program to make it work but it should be seamless afterwards.
I’ve looked and haven’t found any options. I’m a macOS user, excited to see Apple embracing it for Safari finally, so maybe it will be coming to other WebKit browsers in the near future.
I did go through a phase where I created webapps (and I do have webapp manager installed, so I can create a 'webapp' from any bookmark and with any installed browser.
I had a Webapp for BBC Sounds, another for Plex web, but in the end this is just opening a link without the UI. If the UI is an issue, I can just fullscreen it anyway.
Quite often, if I wanted to - for example - open my Gmail, I'd make the terrible mistake of opening the bookmark in the browser instead of opening the webapp.
And you know, quite often I never even noticed I was doing it!!!
As such it was deemed a waste of Firefox resources (because it had plenty of issues).
The only webapp I have now are for individual Google services which need to be opened in Chrome (due to microphone access etc).
In fairness, they never had official support in the first place, what was in Firefox then was a very rudimentary implementation behind an about:config flag.