Since the fireworks are shot off on the Charles River in between the Harvard and Longfellow bridges, either bridge will provide you with awesome views. However, I personally recommend the Harvard Bridge (Mass. Ave.) though, since it is often slightly less crowded than the Longfellow. Bring a blanket or a lawn chair and you'll be all set. Better yet, pick up a box of pastries from Mike's in the North End to enjoy!
Boston's 4th of July fireworks are so big that even from these distances, you'll likely see (and feel) pieces of soot rain down on you. I also agree with a previous post that while the Esplanade can be fun, you have to get there at the crack of dawn to reserve a spot on the lawn....and expect to sit there all day until the concert begins (at 8:30pm). Fireworks are scheduled to go off towards the end of the concert (at 10:30pm).
For more info, see: http://www.july4th.org/