So follow the following steps:
Step 1: Download the add-on click here
Step 2: Now you will see Add To Firefox button click it.
Step 3: Wait few seconds so that till the downloading process finish.
Now you are done. After you install you will see that your browser open your Facebook again. And go to your chat it has come back to the old version. So enjoy see the image below how my Chat looks now :
If you like this page please share with your friends using our sidebar box so that your friends can use the old facebook chat.