Thunderbird is awesome. For a desktop e-mail client. However, HTML e-mail is abused so much I consider it to be the bane of my day.
I recently got a new machine at work and started with a fresh install. This meant my previous settings from Thunderbird didn't migrate with the update and the latest version has had a major refactoring of the layout and menus. Searching for how to turn on plain text e-mail was only returning directions that worked for the previous version and the path through the menus doesn't exist any more.
To reenable plain text e-mail now the easiest option I found was to open the Config Editor from the Preferences > Preferences > Advanced tab. From there search for 'plaintext' and enable (double click to toggle) the mailnews.display.prefer_plaintext setting.
Those e-mails with the coloured backgrounds, visually incompetent layout, headings and colour choices...
Gone.