I just tried to restore my FF profile for the first time, from the most recent of my weekly automatic backups. I discovered that there is no .fbu file inside the dated directories. There are a lot of other things: xpi's, various json files, and in fact an .fbu labeled "permissions" with a 3KB file size. But nothing that looks like the "main" profile .fbu. When backing up manually from the menu, everything seems to go fine - no warnings/errors.
I'm using FEBE 8.9.2 on FF 48.0.2 now, but the backups in question were created with various versions: FF 47, 48 & 49. Am I missing something here?