First of all this idea is related to PWA which as already 1000 votes: Bring back PWA (progressive web apps) - Mozilla Connect
In Google Chrome you can install website as an app with "Create shortcut...", "Open as window". You can do this also in Edge and nowadays they seem to add the website to Apps list of Windows 10/11. This feature has been in Chrome for around 10-15 years? or more but for some reason it seems to not yet be in Firefox.
Although PWA:s are becoming more common there is huge amount of websites out there that are NOT coded as PWA:s but users would still prefer to run them in separate windows and processes with their own icons in taskbar and start menu (and equivalent in MacOS / Linux). Also these apps must be enabled to be run in their own windows without URL, bookmarks, buttons, menus and other unneccessarities near the upper edge.
Although PWA trend is really good thing there are tons of websites out there that will never be modified to be PWA:s. Typical examples are local domestic services, non-profit organizations, etc.. that have limited financial resources for modifying their websites.
A perfect example is an smaller local email service provider from Finland like a vocational school or university of applied sciences or a small niche teleoperator.
It is highly unlikely that they will have funding to develop PWA versions but users need the separare window feature.
Having web apps in their own window is so vital feature that it is impossible to switch to a browser like a Firefox that does not have it.
I give an example. I am using GMail web interface (but same applies to any other like ProtonMail). It is so important app that it must be in its own window and have an own icon in taskbar and not just a tab in web browser. As Firefox does not support this I have GMail app window created with Chrome or Edge. If I click any link in GMail window (Chrome) it will open a tab in Chrome and ignore default browser setting of OS (which was Firefox). The same applies to GMail window (Edge). If clicking any url it always opens Edge and ignores default browser setting. This all ends up to chaotic mess where I have half of work in Firefox and half in another browser. Thus I had to stop using Firefox and switch to a browser that supports web app windows.
Please add this feature asap!