Chrome: Facebook is a bit of a cluttered mess and the messages section is one of the worst offenders. If you use Facebook mostly for emailing people, Distraction Free Facebook Messenger is a Chrome extension that clears out the junk.
Just click the extension's icon and you're taken to a distraction-free version of Facebook messages. The extension gets rid of the top bar, blocks pop-ups, and gets rid of the activity sidebar. What you're left with is a simplified screen where you can just concentrate on sending messages.
Distraction Free Facebook Messenger [Chrome Web Store]