Houston firefighters are standing by for the emergency landing of a of a 50-passenger jet at Bush Intercontinental Airport, an airport spokesman said today. The Continental Express Embraer 145 took off between 4 and 4:20 p.m., said Rich Fernandez, a spokesman for the Houston Airport System. ``It blew a tire on takeoff and is circling the airport now,'' Fernandez said. ``I do not know what airline this is.'' The Embraer 145 is known as a ``regional jet,'' Fernandez said.