thurrott[.]com/cloud/web-browsers/mozilla-firefox/246039/tip-use-firefox-for-web-apps
A web app is basically a site that does something you'd otherwise use an app for [e.g. email], and with current browsers you can set many of these sites up as if they were in fact installed apps. SO far Chrome, and the Chrome-based Edge have gotten all the fanfare, but it works in Firefox too. You can also set up any web site/page in Firefox to work this way, not just special PWA sites.