From their GitHub:

Molly has unique features compared to Signal:

• Data encryption at rest - Protect the database with passphrase encryption
• Secure RAM wiper - Securely shred sensitive data from device memory
• Automatic lock - Lock the app automatically under certain conditions
• Multi-device support -- Link multiple devices, including Android tablets, to a single account
• Block unknown contacts - Block messages and calls from unknown senders for security and anti-spam
• Contact deletion - Allows you to delete contacts and stop sharing your profile
• Disappearing call history - Clear call notifications together with expiring messages
• Debug logs are optional - Android logging can be disabled
• Custom backup scheduling - Choose between a daily or weekly interval and the number of backups to retain
• SOCKS proxy and Tor support - Tunnel app network traffic via proxy and Orbot

Besides that, you will find all the features of Signal plus some minor tweaks and improvements. As with Signal, SMS is not supported.

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Molly is open-source just like Signal. But Signal uses Google's proprietary software to provide some key features.

To support a 100% free and auditable app, Molly comes in two flavors: one with proprietary blobs like Signal and one without. They are called Molly and Molly-FOSS, respectively.