Signed into my account on my dad's laptop. Data was lost and merged with mine?
All his bookmarks and saved passwords were merged into my Firefox account and deleted from his laptop when I signed in from my PC. Is there any way to undo this and return/restore the settings on his laptop?
ప్రత్యుత్తరాలన్నీ (1)
You should lose data if you sign in to Sync, but you should never sign in to your account on a device that isn't your own as that will cause data from both devices to get merged like you noticed. You can try to disconnect from Sync on your dad's device and remove synced data. Best would be to backup your profile data on both devices to be sure.
For the bookmarks there are automatically created backup in the bookmarkbackups folder.