At 1st click the Facebook button in the toolbar and select Remove from Firefox. Then You will see a confirmation dialog asking if you are sure you want to remove the feature. Click Remove Facebook Messenger to completely disable it. After that Messenger should be completely disabled. Then Log in to your Facebook account at facebook.com and Visit facebook.com/ about/messenger- for-firefox then Click the green Turn On button. The messenger sidebar will open and confirmation message will be shown.and Click OK in the confirmation message to turn the feature on.