Imagine a man is running on a treadmill. His inertia with respect to floor will be zero because he is not moving with respect to floor. If both he and the tread mill suddenly stops he will not fall, according to Newton's first law.
Imagine an ant on the conveyor belt. He doesn't worry about our reasoning. He just believes in 1st law, but can't explain it using the 1st law.
(Note: the ant will measure two momentum of the man. same but opposite in direction)
Now imagine we are a 2D being, living in a closed world like the surface of the Earth. Is Newton's first law not applicable here? Then what will be the definition of inertia in this world?
I think I should mention my current level of understanding. I read general relativity only in popular books.