It ultimately solves a phantom problem Rated 1 out of 5 stars

The statement that there is no signature function is wrong. Actually there is a function to automatically insert signatures that few know about. You just click on tools, account options and there is an option right there to automatically include a signature. If the signature is already enabled then the app fails to work. Also, for alternate signatures or switching them the most painless method is to put them into a folder( probably in your pictures) and then just manually delete the default signature, hit insert, image, pick one and put it at the end of the mail. The low rating isn't the spirit of being harsh. The developer appears to have solved a problem that didn't actually exist. Sorry Grufo.

This review is for a previous version of the add-on (0.2). 

hi apollo, add-signature is not designed to do the same the existing signature feature does!
if you start writing an email an you remove the signature there is no function to include it again, you have to copy an rewrite your message.
also if you want to choose when to include a signature to your mail - then you can use add-signature!
there is actually no feature in thunderbird to add the signature to specific mails.