I'm assuming you mean the "From:" box, or maybe the "Reply to:" box ???
The way I do it is to set up multiple accounts within Thunderbird. All use common SMTP server settings but different "From" and "Reply-To" settings.
The latest version of Thunderbird allows you to send incoming mail from all accounts to a common inbox, to avoid having to check each one separately. All versions allow you to use common Sent Items, Draft and Trash boxes.
When you compose a message the "From" box becomes a drop-down where you select the identity you want for that message.
Hope this helps,