Suddenly must sign in with password, first time ever after years of using Thunderbird

Any mail account (email address) regardless of whether imap or pop will always need a password to access the server whether receiving or sending. This is the same for all email clients and also when accessing webmail accounts via a browser. As you have not been manually doing this in the past, that will be because you selected the checkbox for Thunderbird password manager to remember the passwords and use them automatically when accessing the server. Thunderbird stores passwords here: * 'Menu icon' > 'Options' > 'Options' > 'Security' > 'Passwords' tab * click on 'Saved Paswords' button * click on 'Show Passwords' button If you have recently used a product like CCLeaner, then it is possible that it deleted your saved passwords file. I would advise that you do not let products like CCleaner have any access to any Thunderbird file or folder as it can cause issues by deleting useful files. If prompted to enter password, type password then select the checkbox for password manager to remember the password and then click on OK.