You can load a text email from the thunderbird program just file. An html email won't be displayed properly.
You could extract the html body and view the results in a web browser:
cat ~/bin/eml2html
Code:
email="$1"
sed -n '/<html>/,${$!p}' "$email" >/tmp/temp.html
firefox "file:///tmp/temp.html"
rm /tmp/temp.html
I tested this out on an html email I saved using Thunderbird. Copy it to your ~/bin/ directory and run "chmod +x ~/bin/eml2html".
I don't know if this would work on all emails however, but it is worth a try.