hi, go to https://accounts.firefox.com/signup and right-click on a blank section of the page. then go to view page infos > security > view cookies > clear all cookies & try it again afterwards.

Type about:preferences#sync in urlbar .and hit Enter.

This will opens preference page for sync. Here First try to sign with the same email id (with that you tried to made an account in previous),if it is saying that unknown account then you need to create a new account with the email id. Else on your sync preference page, Click on Manage and choose Delete Account . This will delete the current account and you need to create account again.