If you have a relatively modern DSLR where you are comfortable going to ISO 6400, I'd skip the flash.

• Set your white balance manually, to keep the warm, orange tint. If you can shoot RAW and adjust this later, that'll give you a lot of flexibility. If you can set it via Kelvins directly, somewhere around 3500K will probably work well (firelight is oranger than that, but you want to retain some of the look), but best to experiment.

• Unless you have a newer high or mid-range DSLR, your camera will struggle with focus. Plan to manually focus, or possibly to separate focus from the shutter (some cameras allow you to use a different button).

• Wide aperture and long shutter speeds. Don't worry too much about blur — it's part of the visual language we expect from campfire photos anyway. You might want to think about a tripod for some of the shots, because that'll at least separate the motion blur from your own camera shake, but don't be too tied to it or you'll miss the best angles.

If you do you use a flash:

• Don't use that push-on flash modifier. You've correctly described it as a "bouncer", and as that implies, you need to have something to bounce it from. The night sky or trees won't work. It's not big enough to really have any meaningful diffusion on its own, so basically it's just a waste. See When and how to use a push-on flash diffuser? for more on this.

• Do use slow sync — the mode which lets you combine ambient exposure with a flash. You'll have to experiment with this too. More at What is "Dragging the Shutter"?