Unfortunately this isn't a joke for the firemen. They would treat it as a real case until they found the smoke bombs. Even without the worst case scenario of them being occupied with your smoke bombs delaying a response to a real case elsewhere it would still end up annoying the fire department far more than your intended target. They are people charged with protecting human lives and the most valuable property of most people (houses), so I personally think it's extremely bad taste to mess with them.
I don't know how's the situation elsewhere but prank calls leading to the mobilization of the fire department cost quite a nice sum over here, if you get caught.