<Default System Browser> not detected

Being new to NetBeans I first clicked on [Take a Tour] and received the Error message:
"Could not access the URL through the external browser. Check the browser configuration."
First I would have appreciated a hint where this can be done (Tools/Options/General) and
second I wonder why my Firefox is not detected, although it is set as default in the Windows settings under Apps/Standard Apps.
However, when explicitly selecting Firefox in the Options dialog, everythings works fine.