Unfortunately, setting up multiple email identities for email accounts is a bit of editing work. But you only have to do it once.
Use Multiple Identities with an Email Account in Mozilla
To use multiple identities with an email account in Mozilla:
- Open your prefs.js file in a text editor.
- Look for a number of lines starting with "user_pref(mail.identity.id1".
- Highlight all these lines.
- Press Ctrl-C.
- Move the cursor to the beginning of the line just below the lines you just copied.
- Press Ctrl-V.
- Change all occurences of "id1" to "id2" in the copied set.
- If you already have an "id2" set, use the next free number.
- Now change the email address in the "id2.useremail" to the desired address.
- Look for the "mail.account.account1.identities" line.
- Add ",id2" to the existing "id1" string value.
- If the user_pref read '"mail.account.account1.identities", "id1"' before, it should say '"mail.account.account1.identities", "id1,id2"' afterwards, for example.
- Save and close prefs.js.
Now you can send new messages and replies from the new identity easily. Mozilla also selects the appropriate identity in a smart manner automatically depending on which email address the original message was sent to if you reply.

