sudo apt install yubikey-manager
Check output with ykman list. If it displays output like YubiKey 5 Nano (5.4.3) [OTP+FIDO+CCID] Serial: XXXXX then proceed to
the next step. If your Yubikey is not recognized, maybe the following helps: https://support.yubico.com/hc/en-us/articles/360013708900-Using-Your-U2F-YubiKey-with-Linux.
If you see a warning WARNING: PC/SC not available, you need to execute sudo systemctl start pcscd.service and sudo systemctl enable pcscd.service.
Go here https://developers.yubico.com/yubico-piv-tool/ and compile according to instructions. Test it with yubico-piv-tool -astatus.
If it displays Failed to connect to yubikey. and /var/log/syslog displays errors such as Can't claim interface 3/4: LIBUSB_ERROR_BUSY, another application might be causing problems, for me it was ausweisapp2.
After uninstalling ausweisapp2 it worked fine again.
https://developers.yubico.com/yubico-piv-tool/YKCS11/Supported_applications/firefox.html
As of May 2023 the snap installation of Firefox cannot communicate with the module due to some security restriction.
The current workaround is install Firefox manually. After adding the module, it should look like this:
. The certificates should be visible under "View Certificates > Your Certificates"