Easiest Way To Set Gmail/Google Apps As Default Firefox 3 Mail Client

At the Firefox 3 address bar, type about:config and you will be prompted with a warning page as shown below. Just click on I’ll be careful, I promise!
Search for
gecko.handlerService.allowRegisterFromDifferentHost and set to True
and
network.protocol-handler.external.mailto and set to True
copy and past
javascript:window.navigator.registerProtocolHandler("mailto","https://mail.google.com/mail/?extort=mailto&url=%s","GMail")
to address bar and hit enter
for google apps users change example.com to your domain name,
javascript:window.navigator.registerProtocolHandler("mailto","https://mail.google.com/a/example.com/mail/?extsrc=mailto&url=%s","GMail")
Click on add application.
Now go to Tools -> Options -> Applications and search for mailto
Change mailto to GMail
For more details on this check out Addictivetips.com