How to inspect the HTML of an email in Thunderbird
Inspecting the source of an email does not always reveal the plain HTML source, but some encoded byte mess. In order to inspect the HTML anyways, you can use a little trick:
While composing a message, select all (Ctrl + A), then navigate to Insert > HTML on the message window.
If you need to inspect a received message, hit "Reply" to turn it into composition mode.