Final answer:
The use of halon compounds, specifically chlorofluorocarbons (CFCs), in fire extinguishing agents has been discontinued due to their ozone-depleting properties leading to environmental concerns, which led to a phase-out under the Montreal Protocol.
Step-by-step explanation:
The group of fire extinguishing agents that are no longer manufactured due to environmental concerns are the halon compounds, specifically chlorofluorocarbons (CFCs).
These were used for their ability to inhibit the chemical reactions that facilitate combustion; however, they have been shown to be significant ozone-depleting substances.
The Montreal Protocol led to the phase-out of these chemicals because of their harmful impact on the ozone layer and their status as greenhouse gases.
This international treaty was successful in reducing the production and consumption of ozone-depleting substances to help protect the earth's atmosphere. In fire extinguishers, CFCs were mainly used in the form of halons, which are similar in structure but contain bromine atoms in addition to chlorine and fluorine.