I had automatic sync turned on so... "I" didn't do the sync that killed my bookmarks; but Firefox did.

Yes, I do have backups... thanks to EverSync.  :-)   

I was able to recover them, but it was very scary to see almost all my bookmarks gone.

I had the same problem last night. 

In the middle of the night, my bookmark count went from 2300 to 30.

I have automatic sync turned on.  No one was using the computer.  It was password locked.