For those users who do not have any default email client setup on their computer, they can use the new feature in Firefox 3 which lets one set Gmail as  the default email client.

The tip, shared by Gmail blog, will automatically launch Gmail whenever you click on a mailto link.

Here is how the tip goes:

1. Go to Gmail and sign in. While logged in, Run the following script on Firefox by copy/pasting it to Firefox address bar and hitting enter.

   javascript:window.navigator.registerProtocolHandler("mailto","https://mail.google.com/mail/?extsrc=mailto&url=%s","Gmail")

   Click "Add application" when prompted, as seen below 

   

2. Now to set Gmail as default, click this link and on the window that opens, select Gmail and check "Remember my choice" and press OK.

   

3. Thats it!

Now whenever you want to change your default email client in the future, you can do it through Options > Applications and search for mailto.