On iOS, native app are not allowed to ship their own html renderer implementation and therefore have to delegate to the renderer provided by the OS. If that happens, does the markup appear as a view in the app instance, or does the content appear as a tab in the browser? I assume that it's the former. iOS Firefox is famously not gecko but a Firefox-flavored safari and if that would cause markup to appear as tabs in regular safari, FF-iOS would be little more than a URL bar. Each "PWA+" would be its own "safari dressed up as a different app", just like Firefox-flavored safari is.
This was about "PWA+" installed from the app store, individually, just like you can install both FF-iOS and Chrome-iOS (and it's still hardly more than a parody of Microsoft's troubles with forcing internet explorer into win98). Conventional PWA that don't come packaged from the app store appear as tabs or as separate depending on whatever mood Apple had been in the last update cycle I guess.