To disable Facebook Messenger for Firefox, simply right-click the blue F button in the top-right corner of your Firefox window and then select the option to remove it.
“The new Facebook Messenger integration, which is the first new service to come out of Mozilla’s new Social API project, is optional and must be turned on by the user, according to a Nov. 20 post on The Mozilla Blog. So far, the Facebook integration is a preview into the fledgling service, and more capabilities and features will be added.”