my understanding is that a wood furnace is meant to be used with wood pellets or properly seasoned wood. using random rotting decaying dead wood outside probably wouldn't be a great idea - I'll need to look into it more.
Ok, yes that's true, if you burn wet, green or rotten wood it's going to make a lot of smoke. Wood really begins to rot after the first year if it isn't cut, split and properly dried. If you have deadfall around you that you want to use then you need to process it into firewood as quickly as possible and store it somewhere it won't stay wet.