ADB detects fairphone, but fastboot mode not

Yes, in deed. Swipe top to botom and have a look if you see the usb - connection options.
The thing is, you have to UN-check all options. :joy:

edit:
if that also doesn’t work try this on your ubuntu terminal:
$ wget -S -O - http://source.android.com/source/51-android.rules | sed “s//$USER/” | sudo tee >/dev/null /etc/udev/rules.d/51-android.rules; sudo udevadm control --reload-rules
and replace with your computers user you can find in the right uper corner…

source: https://source.android.com/source/initializing.html