Why does FF create/use new profiles when opening link from other apps and how do I prevent it?

Can't find an answer but maybe I use the wrong search terms in which case I beg your pardon and ask for links.

My problem is that I run Firefox portable as standard browser. Currently that is 83.0 (64bit).
All works nice and easy but when another app (e.g. thunderbird but also help in various other software) opens a link it seems to use firefox as it should but it also creates a new profile in c.
In this case firefox works but "my" changes and settings (e.g. bookmarks, extensions etc.) do not. Of course they can't because they are set in the portable profile which is smugly ignored.

Simply deleting such profiles does not work.
Going to about:profiles and setting portableprofile as default/standard does not work either.

As I understand it the programme is firefoxportable (though I can't be sure - unfortunately I installed firefox locally before and am not sure it has been removed completely - there is no "firefox" or "mozilla" anymore in the app list though).
When I check "about firefox" it just shows the most recent version.

How do I force each and every app to use firefoxportable AND the profile there?
And why would it create a new profile anyway?

If there is a hidden firefox app being used instead of ffportable how do I find out?