Why I use the same email address sign up twice?

Yes, I use the Firefox in Ubuntu 16.04. Firstly I used my original Firefox account sign in Firefox, after I entered the identifying code, I successfully sign in and sync. But when I exit the browser and open another time, Firefox tell me I didn't sign in any account! When I type in my original account, it reminds me that I should use this account to sign up a new account, but definitely I have used this address sign up an existing account! The funniest thing is coming! I use the same email address sign up a new account, then there is an identify email coming in my original email account! there is the email of the preceding identifying code！！！ Finally, I use the "new" Firefox account to sign in, but I lose all my bookmarks, passwords, and so on. Could someone deal with this ridiculous problem? The email I used to sign up Firefox is [email]