I never noticed that “Manage Identities” button in Thunderbird’s (Account) settings. I was looking all over this morning for an add-on that would let me spoof my ‘From’ on a reply to a specific account, and not the catch-all account where I was receiving the mail.
This is because use generic email addresses (which I can receive on a catch-all account for my domain, or block outright if they end up on a spam list) when I subscribe to mailing lists / bug trackers. Today, I finally decided I was going to unsubscribe from https://bugs.kde.org/show_bug.cgi?id=92845, but unfortunately the KDE bug-tracker doesn’t allow you to unsubscribe from their web site; you have to send an email from the subscribed address.
Anyway, it turned out that this is a native feature of Thunderbird. Just go to the account’s settings, and click the Manage Identities button to add your mailing lists account name. Then, you will be able to select it (if it’s not selected automatically) when responding to mailing lists.