To my knowledge, every PWA installed from Firefox on Android will become a bookmark. For Firefox I believe that means for example that if you try to open a link elsewhere that is within the manifest scope, it will not open in the PWA. That's because it's not possible to deep link to the PWA without it having an AndroidManifest with a corresponding intent filter, which is what the Chrome WebAPK achieves and why they can support for example custom protocol handlers or share targets or launch handling options.

Other browsers "occasionally" find a way to create a PWA install.

YMMV, it never lasts.

Right now Firefox cant, at least not on any of my android phones.