That's not quite true with the pwa thing. Many of the features of pwa support, particularly the interesting ability to have them work offline, were and are still supported in firefox.
What doesn't work is the ability to view websites as their own "app". This feature was most likely dropped because Firefox had to basically rewrite their UI engine, but now that it's done, we are seeing things like native sidebar (instead of topbar) tabs, and web apps (2025 article) get added again/officially.
Yeah I just did a quick test with photopea.com and it worked offline in firefox.
PostmarketOS is switching to systemd