Was going to post this as a bug, but I'll try here first ...
Thunderbird 24.1.0 Ubuntu 12.04 LTS
(I am assuming since a recent update in the last few days) I find that it is impossible to send an email to more than one receipient. Any attempt to CC, BCC or add multiple 'To:' recipients ends up with a 'dead' send button, and the email. There's no error. Trying to send via File/Send Now or File/Send Later also fails, as does CTRL+RTN.
The bug seems somewhat intermittent. If ccs/bccs are already in the address book and added via the automatic dropdown that appears as you type, the email will send. A 'new' email address that is simply typed into the BCC/CC line results in an email that will not send.
Nothing appears in the error console.
I wonder if this is connected to another bug where clicking on the address book icon in the main client window brings up an empty address book, even though the address book is actually large and available when composing a new email.