How do I setup Mailbird app 2 factor authentication for iCloud email address?

First turn on two-factor authentication for your Apple ID if you haven't done that previously. See: Two-factor authentication for Apple ID - Apple Support

Then get an app-specific password. See: Using app-specific passwords - Apple Support

Then when setting up iCloud email in the Mailbird app, use your Apple ID and use the app-specific password instead of your normal Apple password.

The following has other information that may help: Mail server settings for iCloud email clients - Apple Support