How to prevent Firefox from changing the e-mail address for one recipient

My Firefox address book has the correct (new) e-mail address for John. When I compose an e-mail to John, I use the new address (either from the address book or by typing it in). But when I send the e-mail, Firefox changes my email to John's old address. The sent box shows that the e-mail went to John's old address, and I receive an "undeliverable" message. (At one point, when I started to type John's address, Firefox suggested both the old and new addresses for John. I chose an option to "not show" the old address, but that action didn't fix anything.)