WebApp Sites by Jari Eskelinen

Opens pre-configured sites in a decorationless popup window, making them kind of a "WebApp". Tested under Linux only (Firefox supports PWAs to some extent on Windows anyway, no idea about macOS).

This gives a PWA-like...ish experience. In some aspects it's even better: you browse normally, and when you navigate to one of these sites you get a new chromeless window instead of a tab. Combine this with a desktop environment that can override the window title and icon shown to the user based on the original title (which is a URL), and it works really, really well.

Behaviour seems to be quite nice also when you open a new window or tab or hyperlink from your "webapp" -> a normal Firefox window will open or a new tab will open in an existing Firefox window. It even works if you hop from "WebApp" to "WebApp", though a temporary window opens and immediately
closes.

There is probably some misbehaviour in scenarios I have not tried, so YMMV...

Desktop / menu shortcut cannot be created via this extension, so you need to create a .desktop file by other means.