How to Install Old Facebook Chat Script in Google Chrome
How to Install Old Facebook Chat Script in Mozilla Firefox
After clicking ADD TO FIREFOX button, the popup will ask about installation. Click INSTALL and Its done.
After installing Close the Facebook and Open it again to see the Old Facebook chat is available. if you can’t find, Clear the cache files and try again. Anytime you can remove this script from Gease Monkey add-on to get the New sidebar chat box.