Rated 4 out of 5 stars

I love this option and use it every day for 3 different mailing lists, My biggest issue is that you CANNOT edit the content of something after you have saved it. You must delete the whole thing and rewrite it. NO FUN when something is 5+ MB in size !

This review is for a previous version of the add-on (5.0.1). 

If you double-click on a scheduled draft you can edit it just like any other saved draft and then reschedule it when you're done editing it.

Note that editing a draft removes its scheduled send automatically at the beginning of the edit, to prevent Thunderbird from sending the draft while you are in the process of editing it.