Definitely either closed voluntarily or by order. Copyright for Icehook was last renewed in 2018, so i think they closed a while ago.
IceHook still owns the FireRTC domain ironically, i have an address here:
154 Grand Street,
New York, NEW YORK, 10013, us
It is set to expire next year.
This is different from what is listen on the Icehook website, however there is no trace of Icehook Systems in the offices. There has been a portion blurred from that address however, so that could be their offices.