There is an option under
Tools>options>Advanced>Security Devices. How to use that to encrypt and digitally signed your emails.
I believe you misunderstood the Firefox function.
Firefox itself is not a mail client, therefore it doesn' provide you with ability to encrypt and/or sign your emails; use Mozzila Thunderbird with Enigmail and OpenPGP plugins if you're looking for this kind of functionallity. Here's nice how to: https://securityinabox.org/en/thunderbird_main
The Security Devices function is used for the purpose when you wan't to use different password and certificate manager than the default Firefox one, I believe for example KeePass or something like that.
I think you meant Thunderbird. You can use built-in cryptography features, but first you need your own certificate obtained from CA (to sign emails) or recipient's certificate to encrypt email. Alternatively you can use GnuPG with Enigmail add-on.