What is Messenger for Desktop?
Messenger for Desktop is a wrapper for the official client messenger.com. Therefore it works like a regular browser which can only navigate to the messenger.com web app. MFD doesn't touch your messages, account or personal data. All that is handled securely by Facebook.
Be careful during the installation: https://alternativeto.net/discussions/applications/11718/be-careful-in-the-installation/
Messenger for Desktop Screenshots
Messenger for Desktop information