Pinned Messenger Tab by Logon

Pinned Messenger provides quick, one-click access to Facebook Messenger by creating and managing a dedicated, pinned Messenger tab. When you click the browser action icon, the extension determines if a Messenger tab is already open. If it is, it switches focus to that tab and window or toggles back to your previous tab and window accordingly. If no Messenger tab exists, the extension opens a new pinned Messenger tab.

The extension lets you view and customize the keyboard shortcut used to activate the browser action. By default, this shortcut (Alt+Shift+M, but you can update or reset it through the options page.