Eu descobri com base nesta postagem do StackExchange .
arquivo .muttrc
# .muttrc
auto_view text/html # view html automatically
alternative_order text/plain text/enriched text/html # save html for last
arquivo .mailcap
# .mailcap
text/html; firefox '%s' &; test=test -n "$DISPLAY"; needsterminal;
text/html; lynx -dump %s; nametemplate=%s.html; copiousoutput;