If you are talking about sync, sync is an option. Shut down sync from here.

Type about:accounts<enter> in the address bar. [v57+] https://accounts.firefox.com/settings

You can close the page where you need to set up a mobile device as this is optional. Your account is already created if you have clicked the link in the confirm email. Just keep in mind that Sync isn't meant as a backup, so if you do not have other devices connected as you can read if you click the link on that page then it won't be of much use.

"Why sync multiple devices?"

• https://accounts.firefox.com/sms/why