Thunderbird supports both concepts and templates. If you save a message as draft, it is stored in your concepts folder, but upon completion it is removed from there.

If you save a message as template, it is stored in your templates folder and you can open messages from there and finish the letter and send it. The template stays unchanged in your template folder, so you can easily send it again to other recipients.

Thank you Onno for your reply! Really helpfull for sending new messages. However we reply a lot of email in conversations too. Is it also possible to add a concept or template to an e-mail in a conversation as a reply?

For that you'll probably have to use an extension, like Quicktext or Stationary …

Thank you so much! The extension works, really helpfull and easy!