I have been using this for more than a year and is one of the most productive add-ons on Thunderbird that I have used.
This add-on seems to creating an issue that can be easily fixed. While replying to emails, Thunderbird creates links to images embedded in the original file. While sending these images are attached from the links and sent. The Add-on provides a link to Move the Original message while sending. The Add-on first moves the Original Message and then sends the reply. This invalidates all the links to images in the reply that point to the original mail. Thunderbird tries to attach these images and hangs with the message "Attaching image...". If the sequence is reversed by first sending the email and then moving the original message, this issue will not happen. Hope this can be easily fixed.
This review is for a previous version of the add-on (1.1.5).