Development for this add-on has been discontinued, the original version has been revived and updated, please use that for Thunderbird 78 and higher.
The bug which caused the Message-ID and References headers to be overwritten with <undefined> was fixed.
This add-on allows you to change email subjects.
Changes can be made to the local message database only, or to the message header as well.
It is imap compatible, changes to the header are propagated to the server.
How to use the Add-on:
1- Right click on a message in the message list and select "Edit Message Subject".
2- Use hotkey "E".