Web-based protocol handlers are a new Firefox 3 feature that allow the web-based email application can be used instead of desktop applications for handling mailto: links from other sites. Gmail has been added to the list of mail applications and it's easy to set it as the default email client in Firefox and open all the mailto: links in Gmail, without installing extensions:

• go to Tools > Options
• select the Applications tab
• search for "mailto"
• select Use Gmail from the list of actions associated with the mailto protocol
• click OK to save the settings

Additionally, you can also select Yahoo! Mail or other desktop email clients by this way.