Use Multiple Identities with an Email Account in Mozilla

Identity is an impossible concept to understand already. But multiple identities (of a super-identity?), what are we to think of that?
Fortunately, we don't have to think anything. It's enough that we can use many different email identities with the same account in Mozilla:
If you have multiple email addresses forward to one account, here's how to compose messages and replies using the appropriate email address in Mozilla.