From looking at the diff, it seems to be in the same vein as the IRC, XMPP, Google Talk(!), and some weird network I've never heard of "Odnoklassniki" (https://en.wikipedia.org/wiki/Odnoklassniki)
Fair enough, I guess. I never realized Thunderbird already supported those. I always had HexChat, Pidgin, or Fractal for IRC/GTalk/Matrix at the ready instead.
On the contrary; Thunderbird's chat has built-in IRC support which you can use without installing something like ChatZilla [1][2].
It's not very good, though, including warts such as automatically attempting an SASL login with your credentials, even if you've never registered a services account on the IRC network [3].
https://support.mozilla.org/en-US/kb/instant-messaging-and-c...
So, in other words: it merely expands the already shipping chat clients to be just one more