You cannot see them around you because you are in concrete forest all the time.
If you like to catch one, you can go to Bukit Timah Hill-at night-when darkness falls onto the rainforest, you go along the small path with a small torch light, then you will see one or two green light spot moving around you----that is firflies!!
An alternative choice is Pulau Ubin or Sister Island. Similarly, you need to be there at night, prepare your insect net, and make a good catch!!