window.showModalDialog method has been unavailable when Firefox is running in the multi-process mode codenamed Electrolysis (e10s). At least Office 365 and Exchange 2016 were known to be broken due to an exception thrown by Firefox.
Because it’s technically difficult to support the functionality in e10s and the method has already been deprecated since Firefox 28, Firefox 46 hid
window rather than fixing the error. Office 365 and Exchange 2016 are now working again thanks to their feature detection.
Update: The method is no longer available on Firefox 48 and later as e10s has been enabled by default.