1. Drag a Mozilla firefox shortcut from your desktop, quicklaunch, start menu, or programs list, onto your dock.
2. Right click on your new dock shortcut for Firefox.
3. Select "dock entry properties."
4. In the "arguments" field, type in (or copy & paste) the web address for your yahoo e-mail (ex- "www.mail.yahoo.com")
5. Press the "ok" button.
6. Press the shortcut icon on your dock.
7. Your web browser will open to the page (specified in the previously mentioned "arguments" field.)
I have just tested this with my own web based e-mail. Works like a charm. 