Take an mbox HTML message (e.g. from mutt), rewrite it and send it to firefox so it can be viewed as html.
This is an old public script that I have ported to python 3. See original authors and contributors in script header.
Just pipe the message to the script.
cat message.mbox | viewhtmlmail
From mutt you can create macros like:
macro index <F10> "<pipe-message>~/bin/viewhtmlmail\n" "View HTML in browser"
macro pager <F10> "<pipe-message>~/bin/viewhtmlmail\n" "View HTML in browser"
Use it at your own risk.
GPL v2 or later.