Thunderbird will not allow me to send an email. Send button is not activated in the drop-down menu.

In 'Write' window, the 'Send' button should display on the 'Composition Toolbar'. If the 'Composition Toolbar' is not displaying : 'View > 'Toolbars' > select 'Composition Toolbar'

The 'Send Now' option is also available via the 'Menu Bar' under 'File'. The 'Send' / 'Send Now' button is not activated until you enter an email address in the 'TO', 'Cc' or 'Bcc' fields.

If you have entered a valid email address in the eg: 'To' field and only see an activated 'Send Later' option then it means Thunderbird is in 'Offline' mode. Click on the 'Offline/Online' icon located in the bottom Status bar to swap back to Online mode.