Firefox supports the import of PKCS12 keystore. Therefore, you are going to
"transform" the JKS keystore into a PKCS12 keystore:
keytool -importkeystore -srckeystore clientKeystore.jks -srcstoretype JKS -destkeystore client.pfx -deststoretype PKCS12
Enter destination keystore password:
Re-enter new password:
Enter source keystore password:
Entry for alias clientkey successfully imported.
Import command completed: 1 entries successfully imported, 0 entries failed or cancelled