On Macos Chrome you can still do hamburger menu -> More Tools -> Create Shortcut... and enable "open as window", and Chrome will create a .app file that, when you run it, opens that site as a PWA. Additionally it gets the favicon of the site by default as its app icon (you can change that) and then it shows up in the cmd-tab list as an app completely separate from Chrome. This works really nicely with Slack, especially if you're usually only in a few slack teams. Inside the PWA Chrome apps you can do cmd-N to create a new window if you want to see multiple channels / threads side-by-side from the same slack team. It's a great user experience and pretty much the only reason I keep Chrome around.
Firefox devs removed its PWA / "site-specific browser" support in https://bugzilla.mozilla.org/show_bug.cgi?id=1682593 .