I really love the integrated Facebook messenger in Firefox and use it as my main chat tool for Facebook. Sadly, it has stopped working for about two weeks (maybe longer) on all my PCs with Firefox 20 installed.
The problem was that it only showed the Facebook logo on the upper right corner but anything else was hidden and I did not connect to Facebook. So this is what I did to solve the issue:
First, I deleted all cookies that are related to Facebook. To do this, you must go to “Settings -> privacy -> show cookies -> search for “facebook”” (or something like this, I use the German Firefox so I am not sure how everything is translated). Then select all cookies (click on the first entry, scroll down, hold down the SHIFT-key and click on the last entry) and delete them.
Deactivate and reactivate the messenger. Click on the Facebook icon in the upper right corner in your browser and choose “remove from Firefox”. In addition, type “about:config” in your address bar, confirm the security pop up and search for “social”. Make sure that “social.active” and “social.enabled” are set to false. If they are true, double-click them.
Restart Firefox, visit “Messenger for Firefox”, sign in to Facebook and reactivate the messenger.
Congratulation, everything should work fine now.