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. *https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles For the bookmarks there are automatically created backup in the bookmarkbackups folder. *https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them