Hi Jean-Dominique,are you mixing stable and testing? As far as I can see Firmware-nonfree/20190114-1 isn't available for stable/stretch. Maybe you also need to use a newer kernel. The Debian changelog (https://tracker.debian.org/media/packages/f/firmware-nonfree/changelog-20190114-1) mention "Update to linux-support 4.19.0-1".
Axel
