An infobar should open at the top of the page (between the toolbar area and the page). Click "Add Application". You now can close the Web Console using any of the same methods you used for opening it.
If the infobar does not open and the Web Console displays "Not allowed to register a protocol handler for mailto" then: * In a new tab, type or paste about:config and press Enter/Return to open the preferences editor. Click the button promising to be careful. * In the search box that appears about the list, type mailto and pause while the list is filtered. * If network.protocol-handler.external.mailto is bolded and user set to false, double-click it to toggle its value back to true. * Switch back to your Runbox tab and start this step 2 again.
(3) Assuming the handler is added successfully, the next time you click a mailto link or use Email Link, you should get a list of email options. You can test Runbox Link here and, if all goes well, check the box to make it your default choice.