The upcoming Mozilla Firefox 86 update will drop the app mode for websites, which is also known as SSB or Site-Specific browser. It that allows running a page as an app on your computer with a separate icon on a taskbar, in a dedicated window that will appear in the app list.
How to Enable Site Specific Browser in Firefox
Starting with version Firefox 73, the browser includes a new feature, 'Site Specific Browser', which allows running any web site like a Desktop app, in its own window. It is similar to the Kiosk mode, but doesn't force the selected web page to run fullscreen. Here is how to enable it.