Many years ago there was only one choice of browser and that was Internet Explorer. IE only saved pages in MHT format which is readable only by IE itself. If you have some old IE pages saved but want to open them in Firefox, Mozilla Archive Format is your answer.
Mozilla Archive Format is a simple Firefox extension which allows you to view MHT files within Firefox. Once installed, all you need to do is drag and drop MHT web pages into Firefox and they will render properly to that you can see the content. Mozilla Archive Format also works with to open web pages saved with other browsers as long as they are in MHT format.
Mozilla Archive Format also has the advantage of allowing you to save entire web pages in MHT format. This has a few advantages over Firefox's own built-in save page system as a complete page can be saved as a single file (i.e. web archive), you can name files using the title of the page (title save) and the saved pages are faithful to the original (exact save)
Mozilla Archive Format is an excellent extension for anyone that needs to open MHT files.