Don’t you think it’s frustrating that every time you click on a contact (mailto) link, your Microsoft Outlook will be opened automatically, why you would prefer to write the email with GMail?
Just configure Firefox for this!

Open the inbox of your GMail account.
Write this line in the addressbar:

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

Firefox can ask you to add mail.google.com as an application for your mailto links.\
Now, if you click a mailto: link, try the tips link Firefox 3 will ask which application you want to use.
Choose Gmail, and select “Remember my choice for mailto links” to set the preference permanently.
Go to Tools > Appications > Mailto: set “Use Gmail”

Make sure Firefox is configured correctly in the about:config. (should be by default).

handler.external.mailto = true