[sciurius]

After running several tests I now have solid proof that the Firefly is misbehaving.
At random times it generates a keycode 129 (scancode 161). This code is understood to be a "CD Eject" command. When connected to my laptop, it does, indeed, eject the CD.
Most of the time this code is generated immediately after a pedal press. When the Firefly goes to sleep, and a pedal is pressed, keycode 129 is sent instead of the expected pedal key, which may confuse the software. (As it did in earlier versions os MobileSheetsPro.)
Since keycodes 129 are sent sometimes while pressing pedals, it can not be the 30-minute inactivity timer.
If noone else has a misbehaving Firefly I must conclude that mine is defective and I will trade it in for a BT-106.