About this Add-on
Since 2.0 version in the options:
1- Changes are done into local TB messages database, not into the message (Subject from Header doesn't change).
2- Changes are directly into header, so it is imap compatible
To access Option, since Thunderbird 60:
1- Right click to enable Menu bar
2- Menu "Tools > Add-on Options > EditEmailSubject"
3- Check/uncheck checkbox
1- CRight click on mail, menu "Edit Mail Subject"
2- Hotkey "E"