You can chat with maximum 50 visitors at a time. Visitors can type anywhere on the site and chat among themselves. If you are a visitor on a firefly enabled site then you can start chatting directly without the need to install anything.
Overall, this is certainly a nice concept and introduces a new way of spontaneous communication and interaction with the readers of your blog. You can get feedback from your site visitors directly without any time delay. Moreover even your site visitors get to interact with each other and talk about your site.