Send individual messages from outbox
As you discovered, the Outbox control is all or nothing for messages that are already there. From Mozilla:
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.
So that's a workaround if you have a lot of messages there and want to send a portion of them.
If you want to send an individual message, the Send button is accessed in the message edit window. You need to reopen the message in the message editor to send it. You can do that with ctrl E or from the menu:
Message | Edit as New Message.
Enhancing control over Send Later
In case you aren't aware of it, there is an add-on called
Send Later by Jonathan Kamens (go to Get Add-ons in the Add-ons Manager, and put "Send Later" in the search box). The website is here. It might potentially allow you to avoid the problem by giving you very detailed control over scheduling sending. So you could do things like work on messages and schedule them all to be sent at a specific time and date or after a certain time period has elapsed. This screen shot will give you an idea of the control:
Note that Thunderbird must be running at the scheduled send time. Otherwise, sending will wait until Thunderbird runs again. There are, however, some Internet services that can be used in conjunction with Thunderbird to send messages even if Thunderbird isn't running. The Mozilla link at the top includes some suggestions.