The mortars they are talking about, are the good fireworks. A ball is the shell, has a fuse you light once you drop it in the cardboard tube affixed to a wood or plastic base. Fuse ignites the shell and a propellant charge fires it from the tube. Time fuse gives the big burst after it gets in the air.
UPDATE: The protestors are no longer peaceful as they are now throwing objects, shining lasers, and shooting large fireworks at officers.— Aurora Police Dept. 🇺🇸🇺🇸🇺🇸 (@AuroraPD) July 26, 2020