Fireworks are too small and used too rarely in order for them to cause any lasting damage. A much bigger threat to the air quality is emissions from cars and factories pumping out toxic smoke. If fireworks were to blame, the EPA would be cracking down on them rather than on cars and factories. Yet, we have to put our cars through emissions testing every year and factories face heavy penalties if they do not conform to environmental laws.
Fireworks should not be blamed for decrease in air quality, the blame should purely be pinned on industries that emit toxic fumes. Fire works are used occasionally as opposed to industries that produce on daily basis. You can not compare for example fumes from coal plants with fumes from fireworks.
There are many man-made factors to be taken into account when we look at the effect humanity has had on its environment and the atmosphere of this planet but fireworks are an almost negligible source of damage. A far more likely candidate for blame would be industrial plants and their emissions.
No, fireworks are not to blame for a decrease in air quality. Industrial machinery, cars, and cattle farming are responsible for a large portion of the decrease in air quality. While fireworks may also temporarily reduce air quality, they are not in sufficiently frequent use to really impact the air quality much on their own.