You can auto collapse your recipient fields (To, Cc, Bcc) if your cursor is not on it in Mailbird. Here's what you need to do:
1. Click on the Mailbird menu located at the top left of your Mailbird window and choose "Settings"
2. Go to the Composing tab
3. Check on "Auto-collapse recipient fields on lost focus"
When you checked this option, on lost focus this is what you will see when you compose an email.
When you unchecked it, this is what you will see.