It does work by default in Firefox 3, so I would first look at installed extensions which may be interfering with the normal functionality. As a test, temporarily disable all but ShareThis, restart Firefox, and see what happens.
Whenever I want to email a link from Yahoo mail, it will not make the link clickable. It does in IE7, but not in FF3. I know about the add on of sharethis, but I would like this to work in Yahoo mail. I have color and graphics turned on.