Workaround:

Go to firefox menu Preferences-> Applications and bind mailto: to a wrapper script.

If it's called with a mail address like "APP@wrapper" call the acording application APP, otherwise your mailer. You just need to forward the URI string which was passed as first argument.

Like this something like <a href="mailto:myApp@wrapper?arg1=bla"> should do what you want.

When making this script your OS wide default "mailer", it should do for all browsers.

But better be paranoiac about security issues ... untaint the passed arguments!