Archimede's principle

Hot air balloons, dirigibles and zeppelins use the principle of buoyant force discovered by Archimedes.
They are classified as being “lighter than air”, but it is important to remember that it is not the weight, but the density (weight per unit volume) that is important.
Note that this force, often neglected in accounts of forces, is omnipresent.

In your opinion, how would a Helium balloon behave on the moon?