You will need to save it your hard disk and then install it from Thunderbird Extension Manager. You may try it at your own risk, I will try my best to support any user. Meanwhile it is available here for downloads and updates. I am going to submit it to Mozilla Dev Center and hope they approve it. The 'Send Later' Thunderbird Extension requires to 'Have full, unrestricted access to Thunderbird, and your Computer'. From the limited testing I have been able to do, it works. Scheduling sending an E-Mail with 'Send Later' natively - without an extension Is that possible yet If not, it would be great to get this functionality. The extension saves the message to draft and monitors the messages in the draft folder, when the chosen time arrives it moves the message to unsent and sends unsent messages. So I wrote this extension that extends the Send Later functionality by bringing up a scheduler for the time when the “Send Later” option is chosen (Ctrl+Shift+Enter is the shortcut).

This extension allows you to compose a message and have Thunderbird send it at a specified future time:

Send later thunderbird