Rated 4 out of 5 stars
It was a great extension, but it won't be updated to work with TB 68+. As a workaround though, simply save an email as a file with an .eml extension and then open it in a text editor like Jarte. After you make the changes and save the file, drag and drop it back into TB. Works for me every time.
My four star rating is for what the extension used to be. One star lost because it the extension won't be updated anymore.
Rated 5 out of 5 stars
For no reason that I can come up with, this extension just quit working in the middle of a session. I have it installed in Thunderbird 52.9.1 (32-bit). The option to use it is still there in the context menu, but it's grayed out. Nothing I've attempted will restore it's functionality. When this extension worked, it was wonderful. I was able to remove tons of unwanted stuff from archived emails so that I was able to reduce their size for storage.
(Later) For some reason, the extensions mysteriously started working again. Accordingly, I've revised my rating. Header Tools Lite is a vital keeper.
(Even later) I found out what the problem was. Instead of simply right-clicking on a message's listing in order to get to the context menu, I have to left click on the listing first so as to select it, THEN right click on the selected listing to see the extension un-grayed out in the context menu. I hope that makes sense to others. It's the best I can do to explain the problem I was having.