Portable vs instance already running

I'm using Firefox Portable on Windows 10 and also have one version installed.

When I make D:\files\PortableApps\PortableApps\FirefoxPortable\FirefoxPortable.exe the default program for http etc. I always get a message that already an instance is running if another window is already open.
(If I ignore the message and don't press enter and try to open the link again it is opening in the existing Firefox..)

When I make D:\files\PortableApps\PortableApps\FirefoxPortable\App\Firefox64\firefox.exe the default program it is working. But it is not portable anymore because it uses the profile of my installed Firefox.