Some while ago, Facebook made changes to stop third party access - the likes of Thunderbird using this facility hence why it is not in the available list of recent versions.

As per info from developers of Facebook:

   https://developers.facebook.com/docs/chat

Please correct your features list for the software to correctly reflect Thunderbird's abilities vis a vis chat integration. Everything still says that it integrates with Facebook and includes instructions on how to do so. It is very frustrating to spend time trying to figure out what I'm doing wrong when it is simply a case of gigo from the get go.