How can I view the HTML of a Thunderbird e-mail?

If you're in mail composition mode, which it sounds like you are, forget the "view source" option people are telling you.

You want HTML, not the raw email message with MIME-encoded HTML.

Do This Instead: while composing the message, click Edit->Select All and then click the Insert->HTML menu option.

The window that opens is small, so resize it. Your HTML message will be there (minus headers and body tags). This is more useful to copy a message out as HTML.

Likewise, you can create your message in your favorite HTML editor and insert it using this feature.

Though the menu command is Insert->HTML, it also allows you to edit the message as HTML as long as you select the text before you invoke Insert->HTML.

I often use this when forwarding a message that was a poorly designed email newsletter, and I need to tweak a few things before forwarding it.

You can send yourself a copy of it and then view the raw message using either the menu option View → View Source or the keyboard shortcut Ctrl + U.

As of Thunderbird 31.5.0 this is still relevant, and my current solution is "File -> Save As -> File" and then pick file type "HTML" which does "what you expect". But, you have to bounce out to another application like Notepad or Firefox (view source) to access the HTML source rather than being able to use the built in Firefox text editor, which is not always the most efficient path to your goal.

All the previous answers (view -> view source, other actions -> view source, and Ctrl-U, which are all the same) do not seem to work, because as Tyler Rick correctly notes they present raw message with mime formatting (as well as message headers). And, in the current version, I don't even find View Source available on drafts. Certainly the "view source" implemented is a powerful feature as well, I just think it was that wrong default choice and should be an advanced option.

Use the keyboard shortcut Ctrl + U.