I cant offer you a solution but what i would do personally, change my mail adress, he/she is using your mail adress for the login attempts. After a week or month change it back and see if this is still happening.

No. Do not use the old e-mail ever again.

Make sure you are using a very strong password. If you think you should change the current password;

Resetting the sync password, or changing its e-mail will remove all data from the servers. The username and password are used to create the encryption.

This is a security feature. https://support.mozilla.org/en-US/kb/ive-lost-my-firefox-sync-account-information

• I've lost my Firefox Sync account information - What to do
• I’ve forgotten, or don't have access to, my Firefox Account email address

Note: Only the data on the servers is cleared. The profile data is not changed or removed.