Why not use the Mozilla Archive Format(.maff)?
The only difference between a .htmlz-File and a
.maff-file is that the root directory of the .maff-archive must be empty. So all you have to do is,
- move the files manually up to a first-level directory and change the file extension to .maff
- install the "Mozilla Archive Format, with MHT and Faithful Save" Add-On and associate the file extension .maff with firefox
It works for me