Why has my "Send" button been replaced with "Send Later"
When Thunderbird is running in offline mode, the "Send" button in message composition windows is replaced by "Send Later".
Offline mode is enabled by executing the command File > Offline > Work Offline.
Offline mode may also be enabled automatically by Thunderbird if it detects that your network connection is unavailable.
Therefore, if the "Send" button in your message composition windows has been mysteriously replaced with "Send Later", then Thunderbird thinks you are in offline mode, and you can get your "Send" button back by telling Thunderbird otherwise. Go to File > Offline and check if Work Offline is checked. If it is, then select the menu item to turn it off. If that doesn't work, or if it isn't checked, then try restarting Thunderbird. If that doesn't work either, then make sure your network connection is working.