It's my opinion that a change in the development focus of MAF would be beneficial. Instead of focusing on MHT read, MAF write and MAF read, I believe only when MHT read & write would reach 100% other stuff should be targeted. This way, bug 40873 (regarding RFC 2557 implementation) could be marked as FIXED. You could even earn some money on this, if you take a look at the bug's comments.

With the more recent version of thunderbird, it is already possible to use the "open saved message" option to open .mht files. There are some problems in the display, but this is just a bug in the Mozilla back-end that should not require extensive changes to correct.