MHT is a very useful file format for saving HTML web pages and all embedded graphics into a single file (also known as web archive).
The format is supported on IE, Opera, Microsoft Word, OneNote, etc. and comes handy if you need to attach an entire web page as an email attachment.
Now your favorite browser Firefox 3 cannot export web pages to MHT format by default but there’s an excellent add-in called UnMHT that makes MHT support in Firefox better an IE and Opera.
With Un-MHT, you can read any MHT files in Firefox just like any other HTML or text page.
Other than viewing MHT files, this add-in adds a "Save all tabs as MHT" feature that lets you save the current window or all open websites as separate MHT files in one go taking the file name from title of the web pages.
Considering that Windows Desktop Search can index content stored in .mht files, this may prove very useful if you need to save your work and close the browser.
Related: Display MHT Files in Firefox