"Best" is always a subjective judgment, unless you give some definition of the criteria you consider important in the decision.
Firefox can handle mht files with the Mozilla Archive Format
extension, by the way. I've also read recently that the devs have started seriously focusing on improving it's memory performance.
And Seamonkey is the descendent of the Mozilla/Netscape 6 suite; it is not directly related to Firefox. Although they share a lot of underlying code and concepts, they are separate projects.