Über dieses Add-on
Support for Thunderbird 68 will end soon.
There is no explanation from the Thunderbird Team, but in order to realize "Display the icon as an image." for Thunderbird 78, I had to use WebExtensions Experiments that I do not want to use, which caused "Have full, unrestricted access to Thunderbird, and your computer", but only "Display the icon as an image." is achieved using WebExtensions Experiments.
There are four permissions to use as Thunderbird Web Extension: "messagesRead", "storage", "accountsRead", "downloads".
Frequently Asked Questions
Q: I don't see the icon. The character string such as Message-ID or DKIM-Signature is displayed.
A: This happens if the email does not have an X-Mailer or User-Agent header. Since Display Mail User Agent T identifies the MUA based on X-Mailer or User-Agent in principle, it cannot handle emails that do not have these email headers. Display the message source (☰ → View → Message Source or Ctrl + U) and check it.