Right now only all unsent messages can be sent as specified here: https://superuser.com/questions/1329174/how-to-send-a-message-in-thunderbird-that-is-in-your-outbox and here https://kb.mozillazine.org/Outbox#Sending_mail_in_the_Outbox
You cannot select which messages in the Outbox will be sent. You can only send them all. If you want to send some messages in the Outbox but not others, you could move the others to another folder temporarily.
It would be great if we could right click on an email, or a list of selected emails, in the outbox and have an option to "Send Selected Messages".
I just want to note that even the workaround doesn't work very well because after moving messages to a new folder, they cannot be moved back to the Outbox folder by selecting them and doing "Move To", rather, they have to be opened, one by one, and then click send, and this creates a copy to the outbox which then means that then the message in the folder it was moved to has to also be deleted manually.