In the stated example, a helium balloon does not `float away from the pull of gravity'. It floats up because it is less dense than the air - it's a principle of buoyancy, the same reason an air-filled balloon floats up out of the water but falls down from the sky. The balloon in a car crash will only be thrown backwards relative to the car if there is significantly greater (enough to overcome the balloon's momentum) air pressure in the front than the back. You can perform the experiment yourself - get a helium balloon and go out to a parking lot. Go forward slowly, then slam on the brakes. Repeat the experiment at faster and faster speeds, and see at what point the balloon goes backward instead of forward. I haven't done the math on this, so I don't know what sort of velocities and accelerations you're going to need for this to occur, or if you can attain them in your car. My instinct says that you can't, but I could be wrong.

Oh, and relative to the ground, you're not being thrown forward in a car crash - the car is just stopping quickly and you're not.

A better example of the principle that acceleration and gravity are indistinguishable would be that of a spaceship accelerating in interstellar space, where external gravity sources are negligible, or an accelerating elevator.

In the case of the spaceship in interstellar space, if the ship accelerates at a near constant 9.81 m/s/s, you won't be able to tell the difference between standing in the ship and standing on earth. You will not be able to perform any experiment that will give a diffirent result on the spaceship than it would on earth.

In the case of the elevator, by accelerating it up or down one can alter the percieved force of gravity. Given a sufficiently long elevator shaft, one could, for example, accelerate it downward at a suitable rate to mimic the gravity of the moon. You would not be able to tell if the elevator was sitting on the moon or accelerating in its shaft (until it hits the bottom, or stops accelerating and moves at a constant velocity).