Rated 3 out of 5 stars
This is easy to use and works great, except as it says it works on only the local database. So unlike HeaderToolsLite that changes the actual header subject, the changes here will not show up on other devices you may have that are also running Thunderbird. So if I change a subject for an email in my inbox it only shows up that way on my local machine.
My wishes are that the actual email would be edited instead of just the local database and that this add-on be updated for Thunderbird 68. The ability to edit header subjects is very important to my use of Thunderbird and because v68 has broken this for existing add ons I am currently choosing to stay with v60.
The add-on is now compatible with TB68 as well. In the options you can select, if you want local database storage or global storage (via IMAP) of the modified subject.