It is already possible to view the source code of an e-mail with Thunderbird.
Sometimes I receive e-mails, which have a body/content (= plain text or HTML source code), which is encoded (for example with base64). When I view the source code of such e-mails with Thunderbird, then I see the original encoded source code (= seemingly random letters and numbers, which are useless to a human). I would prefer to see that same source code with Thunderbird, but decoded (= plain text or HTML source code) if it was encoded.
(My current solution for encoded e-mails: I copy the encoded source code and decode it with a different program, but that takes time.)
(I don’t care about decoding encoded e-mail-attachments.)