I have Windows 7 Professional 64-bit and upgraded to Firefox 4 when it came out of beta. I can click on mailto links in IE and it loads Eudora 7.1.0.9 just fine filling out the to and if the link has it, the message as well. Worked ok in previous Firefox versions. In Firefox 4, the mailto links don't do anything. I've gone to Windows control panel and set internet program defaults. Eudora has mailto and send mail. When I go into Firefox 4 options and pick Applications, there is no mailto content type and no option to add types. Anyway to add it? From about:config network.protocol-handler.expose.mailto status: default type: boolean value: false network.protocol-handler.external.mailto status: default type: boolean value: true network.protocol-handler.warn-external.mailto status: default type: boolean value: false Edit for more info: Looks like just the mailto content type isn't the problem. A friend who has Windows 7 Home 64-bit, Firefox 4, and the same (last) version of Eudora has the same problem. However, he DOES have the mailto content type and it is set to Eudora. Still, clicking on mailto links do nothing in Firefox 4 but work just fine in IE 9.