There are a lot of Firefox features that don’t get much press, or are quickly forgotten about. For example, did you know that you could switch to using Gmail or Yahoo! Mail as your default mail client (to handle those mailto links)?
To chance your settings, go to Tools, Options and then click on the Applications tab. From there, type “mail” into the text box to go to the default mail handler settings. From the drop down box, you should get several choices, Yahoo! Mail and Gmail being two of them. Choose the webmail application of your choice, hit the button labeled, “OK” and you are all set.
Now you are ready to use the webmail provider of your choice as your default e-mail application, as far as Firefox knows.