Can I return to my state of an account in Firefox Syns a day ago?
Sorry, Right now we don't have that option
visit the Firefox SYNC FAQ
but you make a backup server daily. I have been there story for a few years.
There is no way to access your Sync account. It is encrypted and we don't restore data. Any backups that happen are only for catastrophic failure of the servers, we can't individually restore data.
STOP PANIC!forgot that I have yesterday's backup.lol
Think a about the daily backup for each user.