You will see a field that is likely empty or set to null . This is where you tell JDownloader exactly which application to launch. You need to enter the executable path followed by a placeholder for the URL.
["C:\\Path\\To\\Your\\Browser.exe", "%s"] jdownloader 2 change default browser
Right-click your browser shortcut on your desktop, go to "Properties," and copy the "Target" field to ensure you have the exact location. You will see a field that is likely empty or set to null