I have had good luck with fireworks handheld. Remember that they are quite bright, although cranking up the ISO helps.
Manual exposure is key. I suggest wide open, if handholding use a shutter speed high enough to handhold without blur, and adjust ISO to get them to the brightness desired. I'd lock focus at infinity-they should be close enough to be there.
With that said, a tripod is definitely helpful, and can go a long way for all sorts of night time photography. Truth be told it's one of the best sharpness improvements you can even in broad daylight, but night time work.
Examples later...I apparently didn't shift most of my good fireworks photos into Dropbox...