If you have already added support for Chrome then all you need to do to add support for Firefox is - go to your project home and navigate to Applications and on the right-hand side click the "Settings" link next to the relevant website. In Push Notification Settings select the Firefox radio button and click save.
If you haven't added support for Chrome please follow the Chrome guide and then select the Firefox radio button.
Once you have set your configs Click save and you are done on the Certs & Keys page for Chrome. Now return to the integration guide, and choose the next browser you need to add, or proceed with deploying the SDK - continue integration.