Add client certificate to keychain on macOS

Most applications on macOS (previously named OS X) uses Keychain Access, the operating system certificate store, to avoid forcing users to add/import the certificate in all applications where it's used.

This procedure describes how to add a client certificate to Keychain Access.

The common reason for having to do this is having used the Firefox web browser to obtain a client certificate and you wish to make it available to other applications (for example Safari and Cyberduck).