Heh, in my country, if any idiot lobs a fire bottle, he will get shot or bayonated.
Rocks, sticks, insults, all are fine on a protest.. they can beat the **** outta you if the rocks are bigger than an egg though.
Lob a rock, thats an aggressive act. Lob a firebomb, thats attempted murder with a weapon... and in any civilized country, if you go into main street in a warm sunny day and start lobbing those things, the police will eventually have to shoot you.