Ask HN: How to View the HTML Part of an Email?

I use Thunderbird. I have configured it so that it shows me the text part of the emails I get.

Unfortunately, every now and then the text part is so broken that I would like to render the HTML part.

But it seems like Thunderbird does not offer a way to do this for a single email.

Looking at the source code of such an email, there is no plain HTML but some gibberish with "Content-Type: multipart/alternative;" or "Content-Transfer-Encoding: quoted-printable".

Any ideas how to go about this?