From MozillaZine Knowledge Base

If you want to have more than two e-mail addresses for an entry in your Address Book, one workaround is to create additonal entries that look very similar (for example, just append an initial or a word like "work" or "home" to the person's name). Then they will be different but adjacent entries and you can insert two addresses for each.

Another way to do this is to open the Address Book, right-click on the name (card) you want to add an e-mail address for, and select "Properties". Enter the additional e-mail address(es) in the "Additional Email" box and click "OK".

To use this, in the "To:" field of a Compose window, type a part of the name you want to select an address to use. (This requires the autocomplete preference to be set to true.) For example, start typing in "John Smith".

Select one of the e-mail addresses from the autocomplete dropdown box that displays after Thunderbird recognizes what is entered with something that is in the address book. For example, with only two addresses for the card:

• JohnSmith <jsmith@somewhere.net>
• JohnSmith <jsmith@elsewhere.net>

With three or more addresses for a single card, you may find that the all addresses you entered into the "Additional Email" box are crammed onto one line in the Compose window dropdown list.