In case of the Sonoff it’s a good idea to go with the Tasmota firmware. There is a whole thread on the Sonoff+Tasmota here in the forum.
You’ll also find a detailed description of how to connect modules to openHAB, with item examples and everything. I’d advice to safe you the trouble and switch right now.
Regarding your problem: first there shouldn’t be a starting slash in an mqtt topic. Seems however to be consistent between your configs. Next issue I’m seeing is that your broker is called “broker” but you are trying to talk to “mosquitto”. One other problem I’m seeing is, that you didn’t provide the login credentials in your mqtt.conf. Is the binding even connected?
Did you read the binding Readme carefully?