Problem
If you leave a Yubikey or similar PKCS#11-compliant physical authentication device connected to the system and have Firefox and/or Thunderbird running, it may continually show a request to “Please enter the password for the PKCS#11 token (name)”.
You can discuss this issue here.
Cause
This is ultimately caused by a bug in OpenSC, which has two fixes pending.
Related Issues
Bugzilla report: #1892137
Workarounds (no longer needed, for reference only)
The easiest workaround may be just to unplug the authentication token when you are not using it.
If you can’t do that, and you don’t use the smartcard feature of your authentication device, you can remove the opensc package to avoid this issue. If you use the smartcard feature but not in affected applications, you can do something like this (as regular user, not root) to disable it in specific affected applications (thanks to Robert Bohne):
mkdir -p ~/.config/pkcs11/modules
cp /usr/share/p11-kit/modules/opensc.module ~/.config/pkcs11/modules/
echo "disable-in: firefox thunderbird vivaldi-bin" >> ~/.config/pkcs11/modules/opensc.module
the values in the last line are executables to disable the feature for; that list covers Firefox, Thunderbird and Vivaldi. You can adjust it for your situation. Then restart the affected applications. If this doesn’t work, you can instead edit the file /usr/share/p11-kit/modules/opensc.module directly, though your edits will be lost each time the opensc package is updated and you will need to re-do them.