I don't understand some of these comments. Start firefox, pick profile, opens a window in the selected profile.

The only weirdness is when you have multiple windows of different profiles opened at the same time and you open a link from a different application it may open it in the last used window which may not be the one you wanted. But then you can of course just drag and drop the tab from one window to the other.

You don't need to switch. Just open whatever profile you want directly.

No need to go to about:profiles. Use a separate shortcut with a -profile option added.