I still use mutt mostly, but it's harder and harder as more emails come in as html - w3m only seems to get you so far...
macro index,pager ,b "<view-attachments>/html<enter><view-mailcap><exit>" "View first HTML attachment in browser"
You'll need to combine it with this entry in .mailcap:
text/html; mutt_bgrun /usr/bin/firefox %s >/dev/null 2>&1; needsterminal