When running an HTTP web server and advertising using mDNS on an ESP8266 device, the service is not discoverable via FlyWeb in Firefox for Android. However, the service *is* discoverable in Firefox Desktop on Mac OS X. This points to a probable bug in the mDNS fallback implementation that Firefox for Android uses since Mac OS X uses the system mDNS responder instead.
This seems to have been resolved by Bug 1296464. Closing.