From the casual user's perspective, it does not "open in Firefox", though. It opens in a separate window with very different chrome, and you have to poke around to even determine which browser is actually hosting it.
Furthermore, on macOS specifically, PWAs are fairly well decoupled from browsers. For example, if you install a PWA in Safari, and then click on an external link in said PWA, it will not open that link in Safari, but rather in whatever your default browser is.
Furthermore, on macOS specifically, PWAs are fairly well decoupled from browsers. For example, if you install a PWA in Safari, and then click on an external link in said PWA, it will not open that link in Safari, but rather in whatever your default browser is.