Ok, friends, I GOT the solution now!
The USB-driver was missing. For some reason, it did not work with this universal USB-driver. Find a detailed description of exactly my problem and solution here:
rooting - fastboot doesn't see device - Android Enthusiasts Stack Exchange
This is the instruction I needed:
ADB and fastboot require different USB drivers.
Whilst you may see your device under adb when your phone is on it will not be detected by fastboot until you install the driver for it and hence will show “waiting for device”.
You can follow the steps below to do so:
- Open your device manager and reboot your device into fast boot by pressing volume up, down and power button simultaneously
- You will notice that device manager refreshes and now your phone has a yellow icon
- Download the Google USB driver, it is a zip file so go ahead and unzip it in your local directory
- Right click on the yellow triangle for your device and update driver, select the option update from local storage and select the folder “usb_driver” under extracted files in previous step
- You are good to go now, fastboot oem unlock will detect the device now
The Google USB-driver worked and immediately fastboot devices recognised the FF3!