Are you using Zoho Mail and want to make it your default mail client in Firefox 3.x? Login to Zoho Mail and copy-paste the following URL in your browser address bar.

javascript:window.navigator.registerProtocolHandler("mailto","http://mail.zoho.com/mail/compose.do?extsrc=mailto&mode=compose&tp=zb&ct=%s","ZMail")

Click on "Add Application" which appears underneath the address bar when prompted from the browser and in the pop-up window that opens, set ZMail as your option.

Now, whenever you click on a mailto:johndoe@abcinc.com link on a web page, it'll open up a Zoho Mail 'Compose' window.