As I understand it, at first Facebook used a standard well-known protocol for their chat application. Some months ago they swapped to something else, of their own design, and this new chat protocol is not supported in Thunderbird. Much as Skype, with its own closed proprietary protocol, is also not supported in Chat.

Having also seen that at one time, Facebook offered an email address to each user, but have since withdrawn this feature, my guess is that they don't want to allow their users to use Facebook services without logging into the Facebook website or mobile app.

Maybe they have your security and safety in mind. I suspect that if you can use Facebook without visiting their website, you won't be exposed to adverts or promotions and that's not good for their business.

I did locate this bug which implies someone is trying to get something up and working, but not at this moment. https://bugzilla.mozilla.org/show_bug.cgi?id=1141674

Ok, thanks for the help.