Even uber-cheap digi's can take good fireworks shots, the absolute most important thing is to keep the camera steady. So yes, a tripod is neccessary unless you can find a garbage can or something else to hold the cam absolutely still.
As far as what to do with your camera, I recommend a remote shutter release, though I'm not sure if the RebXT supports one.
Run the camera in either full manual or shutter priority and set the shutter for anywhere from 1\2 sec to 4 seconds, you can go anywhere between to get the effect you like. In shutter priority, the cam will automatically adjust the aperature. In full manual, you will need to adjust it manually. I would recommend starting around f/8 and go up from there, adjust the shutter to compensate for the aperature.
Also, set your ISO fairly low, 50 or 100 would be a good place to start, this will help eliminate any noise that will become present in dark exposures. Lastly, don't use the flash, it will do absolutely nothing but turn the pic into garbage.
HTH