Web mail notifier is a Firefox add-on where you can configure your Gmail, yahoo and Hotmail accounts and this add-on will notify you where there is a new mail. Go to this page and add this add-on to your Firefox – https://addons.mozilla.org/en-US/firefox/addon/webmail-notifier/
When you have done with the installation, restart your browser to see the application in your browser as shown below. Right click on this and go to options,
Select Gmail from the dropdown and enter your username and password and click add button, then select yahoo and enter your username and password and click add button, likewise hotmail etc.
When these accounts are added and a new mail comes for any of these accounts you get an alert as shown below.