If you are using Mozilla Firefox, Google Chrome or Opera as browser then, there’s a solution to get your Facebook old chat system back.

You just need to install ‘Sidebar Disabler’ extension on your browser. With this extension you can disable the newly introduced Chat Sidebar and return to the old Facebook Chat. Enjoy!

This guide is very simple, just basically involves downloading and installing an app.

Lets start off with google chrome:

Navigate to the the chrome addon site

Facebook Chat Sidebar Disabler For Chrome

Click install and your done, you dont have to like that facebook page that pops up but it helps the creator.

Next for Firefox navigate to
Facebook Chat Sidebar Disabler For Firefox

Install, let it do its thing than restart Firefox and your good :)

For Opera same thing, navigate to
Facebook Chat Sidebar Disabler For Opera

As of now there is no way to get the old chat back on internet explorer, when there is i will update this thread but for the mean time you will have to use one of the three browers.
Leave your feedback and a Thanks is appreciated.

Tips:

-Make sure your browser is up-to-date.
-Restart your browser after installing the script
-Download the scripts for Firefox from Firefox itself, not from any other browser. Same for Chrome.
-Not working? Try un-installing the script and then installing it again.
-Make sure you do not have more than 1 copies of the same script installed in your browser, as it may cause conflicts. Always remove previously installed script before re-installing it.