Delete all Cookies at once

Most of the people have more-than one email accounts and any other accounts. So if you want log-in every accounts you may face some log-in errors. At that time you have to clear all your existing cookies. 
And also if your PC has lots of cookies your web browser speed will reduce. 

Cookies are small text files that store your log-in data and your web page visiting data.  

You can do in Firefox by clicking "Tools>Clear recent history". But now you can do it installing Firefox add-ons called "Cookies Monster".

Get cookies monster in Firefox official Add-ons web site.

After you install it you can view link at bottom-right corner of window, when you click on "Cookies monster" link you can view "Clear cookies" button, click it for clear cookies. 

After all cookies delete dialog box will appear and indicating "All cookies are deleted" 

This is easy and fast way for clear cookies.