According to https://www.optimum.net/FAQ/#/answers/a_id/216 you need a username and password to access your Optimum email account either via IMAP or POP and to send via SMTP. If you never had to use a password before, now you do, and if you were using insecure connections to the servers, your provider now recommends you use secure connections (SSL encrypted). Reset your password if you've forgotten it, via webmail or the reset password facility provided by your provider.

The primary (formerly 'master') password is for guarding access to the account passwords. If you're asked for one and have never set one, Help/More Troubleshooting, Profile Folder, Open Folder, close TB, delete or rename key4.db, logins.json, cert9.db. This will erase all account and primary passwords. When TB restarts, enter the account password when prompted.

http://kb.mozillazine.org/Master_password#Resetting_the_master_password

You can also use the Error Console, Ctrl+Shift+J, and enter chrome://pippki/content/resetpassword.xhtml