It's an additional function for Thunderbird.
I'm not sure the world is in dire need of another messenging protocol, but here it is. I'll hold off until they add it to Pidgin, thank you. I'm not using Thunderbird. There ARE a LOT of T-Bird users, though, and for all I know they are clamoring to have this. It's from the Mozilla folks, so perhaps there is less worry about security and privacy than there might be with other vendors. And the price is right.
Check it out.