Had the same problem, trying to sign documents with libreoffice, and used the same method to set the certificates store path in libreoffice (tools / options / security / certificate button, and set the path (manual) to: ~/.pki/nssdb
Now I can manage my certificates with google chrome (no snap version); firefox and thunderbird and chromium snap have their own certificate databases.
Had the same problem, trying to sign documents with libreoffice, and used the same method to set the certificates store path in libreoffice (tools / options / security / certificate button, and set the path (manual) to: ~/.pki/nssdb
Now I can manage my certificates with google chrome (no snap version); firefox and thunderbird and chromium snap have their own certificate databases.