What OS? What version of Firefox? Lets start from the beginning and provide much more information please.

Is this a problem with connecting to Sync to restore your previous personal data ?

Note that resetting the password for the Firefox Account will remove all data stored on the Sync server because this password is used to encrypt your data before it is uploaded to the Sync server and only this password or a possible recovery key can decrypt this data.

• https://support.mozilla.org/en-US/kb/ive-lost-my-firefox-sync-account-information