If you use Yahoo account with 2 Factor Authentication, you need to create and App password for Mailbird.
1. Go to Account info and click "Account security" on the left
2. Click "Manage app passwords"
3. Select "Other app" and put "Mailbird"
4. The new password will be generated
Then add your account in Mailbird normally via Settings -> Accounts -> Add. Paste the app password in the Password field in Mailbird.