Facebook deprecated their XMPP Chat API for whatever evil business reasons they had. As their documentation and various blog posts or even StackExchange questions say, accessing Facebook Chat in that way became impossible. I found that out myself too as my Pidgin at work stopped working with my Facebook account.
Now comes the strange part: at home, Thunderbird still can connect to my Facebook account. As far as I could Google Thunderbird also uses XMPP to communicate with Facebook servers, and they don't have plans (or time and manpower) to change this. How is this possible? How is Thunderbird different that it still can connect to Facebook?