As edlin suggested the answer is in the warning messages:
Line 2 in my debian.list is:
deb
https://deb.debian.org/debian/ unstable main contrib non-free
line 11 which I added at michaa7's suggestion is:
deb
http://deb.debian.org/debian/ unstable main contrib non-free non-free-firmware
With line 2 uncommented my firmware doesn't doesn't update.
However; after commenting out line 2 and uncommenting line line 11, my firmware updates correctly with a dist-upgrade.
Problem solved!
Thank you all for your help!