Thunderbird stopped working with iCloud email
Starting June 15, 2017, Apple requires two-factor authentication on all iCloud accounts that access their accounts non-natively. Apple’s third-party email client setup instructions have since been updated accordingly.

Native iCloud access is limited to System Preferences on macOS, the Settings app on iOS, and iCloud for Windows. All other apps, including Thunderbird and Outlook, require an app-specific password.

To enable two-factor authentication:


On an iOS device open Settings → Password &amp; Security and tap Turn On Two-Factor Authentication.
On your Mac open  → System Preferences → iCloud → Account Details → Security and click Turn On Two-Factor Authentication….


Then, login at https://id.apple.com/ and under Security → App-Specific Passwords, click Generate Password…. Enter that password into Thunderbird.

This was done to keep third-party apps from getting access to your iCloud password, thus reducing the likelihood of iCloud account breaches.