Save Web pages in MHT & MAFF using Firefox
1) Download the extension file
2) Rename the file extension from xpi to zip.
3) Open the zip file
4) Open install.rdf
5) Change MinVersion to Current Version of Firefox version
6) Change Maxversion to 1.0 (or higher)
7) Rename zip back to xpi
8) Drag xpi to your firefox.
You can download from here.