I just switched to Firefox and struggled with this exact functionality. The posted site ships a script for curl|sh that creates a new firefox profile with a custom userChrome.css.
I ended up writing a firefox extension to open up URLs in a new "popup" window[1]: https://addons.mozilla.org/en-US/firefox/addon/web-app-maker.... I just checked it on Windows (usually using linux) and the popup window type does not allow the maximize button to work. Oh well...
There's an excellent extension that makes this pretty simple to manage. Additionally the windows it creates have some pwa specific settings, like how links should be handled.
https://github.com/filips123/PWAsForFirefox/