Direct firing my mash tun while recirculating has been a huge improvement to my brewery. Efficiency went WAY up. Without recirculating with that full width false bottom, the wort underneath superheats and will boil (learned that the hard way when I killed my pump with the burner on). It didn't scorch the grain, and the bulk of the grain bed didn't get anywhere near boiling, but there was some scorching of the wort. Without a pump, I would not direct fire my mash. If you have a domed false bottom that doesn't cover the entire bottom, near constant stirring may work, but sounds like a lot of effort. Personally I'd stick with a cooler at that point.