Rated 3 out of 5 stars

This is a useful and critical add-on for me, but the author doesn't seem inclined to update it for multiprocess AND the new WebExtensions format. If he wants us to Donate toward the work, it is not obvious.
In the meantime, I've discovered that you can
a) install the 'Chrome Store Foxified' add-on, then
b) visit the Chrome Web Store,
c) search for MHT and finally
d) install to Firefox the Chrome add-on named 'Save As MHT'.
So far, it seems to work. Crossing fingers .....

This review is for a previous version of the add-on (4.1.1).