Many Thunderbird users work with "Plain text" or "Simple HTML" to display email in message reader. Only for some messages it's necessary to have "Original HTML" enabled to read them. In core Thunderbird Its necessary to first enable this manually in menu and after reading the message to manually switch the "Original HTML" option back to "Plain text" or "Simple HTML" accordingly to what the user wants as his personally default. This is uncomfortable and there is "a risk" to forget to do this before opening an other message.
To provide a more comfortable way I created the "Allow HTML Temp" add-on years ago:
Listing on ATN: https://addons.thunderbird.net/en-US/thunderbird/addon/allow-html-temp/
AFAIK, this add-on will not be possible as a Web-/MailExtension because of manipulating directly some Thunderbird core prefs in the background.
We should implement the main feature into Thunderbirds core to show the selected/displayed message once with original HTML and then directly switch the Plaintext/SimpleHTML/OriginalHTML option back to the users prior chosen default. As an improvement over this, the decision to show this message with original HTML should be saved/persisted the same way as we yet do for external content. By saving the decision, it would be possible to improve the print and answer/forward message UX.
I believe this feature would be outstanding from other mail clients.