An update for anybody else looking at this problem.
TAC have confirmed the following:
The .networkconfig file is served from Onboard using the MIME type arubanetworks/networkconfig. Our QuickConnect application registers for this MIME type with the Android system. The Android native browser honors this MIME type association and correctly launches the application when the file is received. Non-native browsers are not able to do this.
We suggest you to use Android native browser to avoid this issue.
So Firefox can' t be used to onboard an android device.