I don't think you quite understand how PWAs work. "PWA" doesn't really mean anything. It is just the idea of a website that behaves like people expect of a native app. Offline support is generally accomplished via the Service Worker API which Firefox fully supports. I have offline "PWA"s on my home screen right now. But these APIs don't actually depend on anything about the home screen. Even just navigating to an offline-capable "PWA" via URL should just work in Firefox.

Maybe some sites special case Chrome, but that is a limitation of the site, not any limitation of Firefox.

One collary as that with an ever growing set of APIs associated with PWA there is going to be some features that differ between browsers. For example Firefox currently doesn't support receiving shares but Chrome does. But I can't think of any "fundamental" PWA API that Firefox on Android doesn't support.