I think your Analysis folder is not being accessed, try:
# also notice the double slashes throughout
env.workspace = "H:\\Data"
input = env.workspace +"\\Analysis\\mtbs_state_2000\\al.shp"
output = env.workspace +"\\Analysis\\2000_dissolved\\al.shp"
statistics_fields = [["km2", "SUM"], ["HU90", "SUM"]]
arcpy.Dissolve_management(input, output, "Fire_ID", statistics_fields)