As far as focusing goes, I usually focus as I would for a landscape... now, technically i think that's supposed to be about 1/3 of the way into the scene. I'm not sure that I follow that, but what I do know is that I focus in the same way I would for a landscape to be in focus. I use a tripod and remote. I set my camera's ISO as low as possible and set the aperture around f/16 but I adjust it based on whether it looks like things are too bright or too dark. For example, something like this:
| Camera: | Nikon D50 |
| Exposure: | 1.7 sec (17/10) |
| Aperture: | f/16 |
| Focal Length: | 18 mm |
| ISO Speed: | 400 |
Because it was really dark out there and there were no city lights in the background or anything I set my ISO to 400 so I could get things bright enough and still get the firework streaks. I also usually set my shutter speed to bulb and open the shutter when I hear the firework being fired and shut it after the fireworks have streaked a bit.
And if you haven't already seen it, try
How to Photograph Fireworks Displays on the blog.