At that distance, I would think you could focus on infinity (then back it off just a touch... as seems to be the case with any lens when focusing to infinity). I'm hopefully going to be able to capture some fireworks tonight but have never tried before. I'm also counting on getting there early enough that I catch their test shots, too. Seems that every fireworks display I've ever been to does this... you know, to gauge the wind and stuff to make sure it's safe to proceed. Hopefully I'll be able to use their tests to frame and focus the shots for later in the evening.
Make sure you post your pics... good or bad... then tell us what you would do differently. Something tells me there's going to be a flood of fireworks pictures. What with Canada Day on the first and of course Independence Day on the fourth. By Monday we should all be sick of seeing fireworks.
Good luck!