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.