Well congrats
Now define a very simple testing item:
Switch MQTT_Test "Testing..." { mqtt="<[broker:testing/mqtt/topic:state:default]" }
With this one in place you should be able to do a first tst. Use your mqtt.fx to publish the Message “ON” to Topic “testing/mqtt/topic”. Check the log once again!
How did we do this just now? See: http://docs.openhab.org/addons/bindings/mqtt1/readme.html#item-configuration-for-inbound-messages
If everything works as expected you can now start to define specific items for your Sonoff. Once again, don’t bother with Espeasy. Go Tasmota: