There’s a new very useful feature in Firefox 3.0.3 that enables you to open mailto: links on web pages in the email program of your choice.
For instance, you may open email links directly inside Gmail, Yahoo! Mail or you could configure mailto link with desktop based email clients like Outlook, Windows Live Mail or Thunderbird.
To access this option, go to the "Tools" menu, click "Options", select the "Applications" tab and then search for "mailto" in the Content Type column.
Now when you click an email link on a web page, it will directly open a "Compose New Mail" window of Gmail or Yahoo! irrespective of what your default email client is.