I want to rise the same issue. Is there a flag in config we can switch? I think the default profile always opens external links, no matter what (even if a window of that profile is not open)

Yes, only the Default Profile will be used by external links. If the Default Profile isn't running that Profile will open it in a new window.

I don't see a "switch" to change that - the current Profile Manager has worked like this since it replaced an older Profile Manager back in Firefox 0.9 in 2004. Don't recall the specifics with that old 0.8 an earlier PM with external links. though. Plus the current -no-remote command line "switch" came about in 2007 to take the place of an operating system "environment MOZ- variable command" that was being used on Windows OS before -no-remote was added to Firefox.

I am expecting a "new" method of "managing Profiles" to come to Firefox in the future, but not any time soon. With the current PM being 14 years old, and a different "external PM" that was being worked on in alpha and then beta never making it to the release stage back in 2010 - I'd be surprised to see anything "new" withong the next couple of years. I hope it comes sooner, but "time will tell".

A 'normal' user-friendly profile manager and this options is something I really miss when I went back to Firefox from Chrome.

Are you aware of any documentation regarding the profile manager? I would love to dig in and maybe come up with an extension that is able to emulate what the Chrome profile switcher does for starters!