Every time I'm trying to save some pages it doesn't works, and my Browser Console gets spammed with some Telemetry errors. I have telemetry completely disabled in my browser. At first I thought that's just the problem with some resources that extension trying to fetch from saved pages, but it happens almost every time. I'm not enabling telemetry for this.

It still light-years away from MAFF/UnMHT in terms of reliability, and it seems nothing's gonna change because Mozilla is too lazy/stubborn to add native support or at least proper APIs for something like .mht format.