Hello. Using Sync is optional, so there is no obligation to use it. Sync is designed to be used to sync your personal data between all devices that are connected to this Firefox Account and isn't meant to be used a a backup, so if you would use is as a backup to restore data then no success is guaranteed. Once you reach the page to add another device, you have successfully installed Sync on your device and you can close the tab where you can add more devices and start using Firefox.