Comment 2 for bug 432285

Can you please attach the output of the following commands:
apt-cache policy thunderbird mozilla-thunderbird
lsb_release -a
uname -a

Do you have any extensions/add-ons/ themes installed, if so what are they?

Please make sure to check preferences menu (edit->preferences->composition->general->send options the first section should have an ask me what to do.
I have it checked and it asked me how i would like to send it. You can also choose to always send HTML or plain text and send as both.
What did you have set there by default or at least when you filed this bug.