The solution I use: turn on Airplane mode, then try to load the site. Firefox will, of course, be unable to load it, but you can still add a shortcut to the homescreen. Doing it this way means you block the PWA manifest that it's loading that takes away all the reasons you loaded it in a web win the first place.

If that doesn't work, you can also put a rule in uBlock to block it, clear all the caches/cookies, and try again. If you need that rule let me know I will grab it.