How to delete all messages on facebook quickly

I tried many methods to delete multiple messages at a time but none worked fine. Using this method messages cannot be deleted all at a time but they can be deleted very quickly.

1)
 Install firefox if you don’t have on your pc. Firefox can be downloaded from http://www.mozilla.org/en-US/firefox/new/

Go here

2)
 Install Gease monkey Add-on. You can find it at the below link
https://addons.mozilla.org/en-US/firefox/addon/greasemonkey/

3)
Click on "Add to Firefox" to install the addon.
4)
Restart firefox
5)
Go to userscripts.org and search for "facebook messages"

6)
Select "Delete facebook messages"

7)
Click on Install

8)
Click Install

9)
Restart Firefox

10)
Open Facebook and go to messages. Now you will find Delete Messages option next to message search box. Click on it.

 11)
Click on Delete. Now you will notice that you can quickly delete fb messages

12)