You can go to about:profiles and save it to your favorites bar. Boom - instant profile switcher. I put the bookmark in the top left of my bookmarks bar on all my profiles and use it to switch between them.

You mean the -ProfileManager (-P) flag? That dialog has been there for a long time. I think in some situations it gets displayed automatically at startup and that’s how I discovered it.

You can just install an addon for that but it’d be great if that would come with ff, I agree.