That is weird. The opposite: i.e. you get only the HTML file and not the _files folder would make much more sense.

What Firefox locale (user interface language) are you using?

Thanks for reply.

In arrachment i has added screenshots with "About" and with Plugins (additional modules). I'm using last version (autoupdate and also on site reported what "... You using last version of ..."). Maybe russian variant, 64 bit (at any case it shows its menu on russian). In similar configuration (and last (and long enough) time - practically in the same) Firefox is working already many years but reinstalled some times (last times - at summer 2016).

But really ".htm" not saving now anyway.