It's been a long since I used mutt regularly, but I recall that I had my mailcap setup to pipe the HTML straight to FF when I couldn't read the text version. It worked pretty well as long as images weren't embedded in the email.
I personally use sup, but that's exactly how I did it. All you have to do is add a small hook, explained on a multitude of Mutt wiki pages. What issue did you have with images? I just have images set to xdg-open my picture viewer.