One of the things that have irked me with Thunderbird from the beginning was its insistence of defaulting to HTML e-mails. E-mail for me is about the text, not about formatting and even less about embedding pictures and what have you. But Thunderbird descends from Netscape Communicator who was the pioneer in this annoying HTML stuff. :-(
There has always been an option to default to plain text but it has moved around the configuration basically with every update. The latest version has got it under Tools/Account settings. There you can – separately for each account – under Composition & Adressing, set the option “Compose messages in HTML format”, or rather unset that option.
I hope this will be the last time I have to hunt for it.