Thanks, but a much wider edit window would be even more useful Rated 4 out of 5 stars

I have been looking for an add-on that will do this and am very glad to have it. However, the editing window needs to be three or four times as long before it will be really useful to me; I need to be able to see the whole subject line to make editing easy. I tried to drag the edge of the dialog box to achieve this, but it was a set size.

This review is for a previous version of the add-on (1.0). 

New version

New Version 1.0.1 waiting for review: https://addons.mozilla.org/fr/thunderbird/addon/edit-email-subject/versions/

With wider and resizable edit window