[forty]

How do you choose in which profile links from outside of the browser should open?

[Dudemeister_]

This depends on what profile you make the the default startup profile in the profile manager

[forty]

Do you know if there is a way to be prompted each time?

[sp332]

You could find where in your desktop environment it decides which app to open, and replace "firefox" with "firefox --profilemanager".

[case]

If you’re on a Mac, Choosy is a great app for this:

https://www.choosyosx.com/

I have no affiliation with them, just a long-time happy user.