In the book Spacetime Physics by Taylor and Wheeler, a free-float reference frame is defined as
a reference frame where every free particle initially at rest with respect to that frame remains at rest, and every free particle initially in motion with respect to that frame continues its motion without any change in speed or direction.
That is the same definition as an inertial reference frame.
The authors went on to say that a room falling towards Earth is a free-float reference frame and hence an inertial reference frame.
However, I also learned previously that inertial frames move at constant speed relative to each other. Does that mean that the reference frame of a stationary observer on Earth is not an inertial reference frame?