window.showModalDialog method, deprecated since Firefox 28 and disabled since Firefox 46 when the browser is running in the multi-process mode dubbed e10s, is no longer available on Firefox 48 and later as e10s has been enabled by default.
Not all Firefox users will benefit from e10s as of the release of Firefox 48, and technically the method still exists when e10s is disabled. However, given that it’s practically gone, you should be using the standard
window.open method or a custom in-page modal UI instead. In the latter case, it is recommended to implement the WAI-ARIA
dialog role to ensure the accessibility. Unfortunately the HTML
<dialog> element is not yet implemented in Firefox.
Update: The method has been removed from Firefox 56 for all users.