Works nice and easy with light single-page MHT files. Doesn't work with a multi-tab, crossreferenced MHT file, saved by old UNMHT addon, saying the adress wasn't understood whichever reference is clicked on.
The file works without any problems on old version of Waterfox however, which is the one I'm using just for viewing MHT files so far.

I have a bunch of old webpages saved as MHT, and while Thunderbird is able to show some of them, it does not work with every file. This MHT viewer lets me view all the missing pages, thanks a lot :)