The same story as story of every other city.
A town which was existing in Southern part of Firozabad since 10th Century which was being ruled by King Chandrasen.
Later in 17th...
more... Century, Firoz Shah settled around 2 KM north from ChandraNagar and given the name to that town Firozabad.
Both towns were separate initially but as population get increased, both joined together and as usual when 2 different towns get merged, the name which is having more popularity get identification in future.
Some part of Firozabad City are still having names like Chandrawar Gate, however Central part of Firozabad known with history related with FirozShah only.
Suhag Nagari is another nickname of Firozabad because an another sub town Suhag Nagar which was developed later become a hub of glass bangles manufacturing industries. As soon as Bangles factories and other Glass items(Pearls, beeds, Chandelier, Paperweights, handicrafts etc.) producing Industries spreaded in entire city, Suhag Nagari name came in effect.