A Note on Firecracker's Noise
Firecrackers may easily produce very high sound levels, and the noise they cause is of an impulsive nature, which means that it has a very short risetime, i.e., a very rapid onset. The human ear has several "built-in" protections against noise, but none of them is capable of coping with that kind of noise. Those natural protective mechanisms require rather long times to operate. For instance, there is a muscle in the middle ear which after some 200 milliseconds of an intense sound reacts reducing the transmission of sound towards the inner ear. However, 200 milliseconds is too late when impulse noise is involved. This in turn means that without an adequate artificial hearing protection, the ear is exposed to a very dangerous stimulus. This might cause permanent hearing damage ranging from tinnitus (ear ringing) to several levels of deafness.
As to sound levels, usual figures rise to 150 dB or more, the same as guns. The usual way of measuring this kind of noise such that the result is meaningful, is to use the C frequency weighting (i.e., to read the output of your sound level meter in dBC instead of dBA as is the usual practice when measuring other kinds of noise), and to use "peak" time setting. If your sound level meter doesn't have a "peak" position, pick "fast" (or "F") and add 10 dB to the reading.
The World Health Organisation suggests that impulse noises should be not more than 140 dB peak at the ear. But in the section on toys, the criterion is 130 dB at the ear of child. I would take this criterion, since it is almost sure that children will be present while using these devices. However, most sound level meters won't measure that high, so you might do this: place the firecracker in an open place, such as a square or a field, and measure the noise it produces at a proper distance (for example, 50 m or 150 feet, or as required to get a meaningful value). Then add 6 dB for each halving of the distance until you get the distance you would usually use the firecracker. Suppose, for example, that you use it at 10 feet, then you must divide roughly 4 times by 2, so you must add 24 dB.
If this is under 130 dB, it might be acceptable. However, as you cannot be sure that all users will use these devices at the same distance, and to rule out accidents, I would ban out any firecracker which produces more than 130 dB peak at a distance of 2 feet, should it accidentally explode near the user's hand--you wouldn't like that at the same very instant a guy lost both a couple of fingers and the sense of hearing :( . Of course, you cannot be sure either that people will be wearing hearing protectors while playing with firecrackers--rather, most likely they won't, otherwise what is all this stuff of fireworks about?
Another suggestion would be to make it unlawful the use of firecrackers except at a few very traditional dates--such us the Independence's day, or the like.
But if it depended on myself only, I would rule out every kind of firecrackers. First, they are quite hazardous, from the point of view of both physical and hearing injury. Second, their noise resemble very closely gun shots, and due to complex psychological mechanisms, this sound becomes a familiar and desirable one, which in turn is hand and glove with an increasing disregard for life. Third, they are used under the (mis)assumption that in order that joy exists you need noise, very loud noise. This is a misconception, in my opinion. You may be very happy, you may be enjoying yourself a lot, without the need of loud noise (even painfully loud) around you.