.mht files. What are they and how I opened one in Linux

I received an email today with an attachment. However, it was a file type I was not at all familiar with (I don't use Windows).
I was puzzled when I couldn't get it to open completely (although some stuff did appear when I tried to open it with Opera 10 Beta 2 but not enough to make sense of the message).
Puzzling as this link suggests these file types should open in Opera.
This blog post seemed very promising but didn't work at all for me possibly because it was meant for a much older version of Firefox.
Finally I got it to open by installing the Unmht addon in Firefox 3.0.12.