Yakalike is one of many chat-based extensions available for Firefox. It gives you the ability to chat with other people that happen to be on the same web page you are on, as long as they are also using Yakalike.
The first thing you may notice once you have installed Yakalike is that you have a new button added to the bottom right hand corner of your main browser window labeled "Yakalike".
Clicking on this button will open the Yakalike main window, located at the bottom of your browser window. Once the main window is open, clicking this button again will hide the window.