The Starmaker writes:

Come onnn already, ...you can feel 'fear of falling' in a "secure
place?" like "inside an airplane??"

A fear of falling usually presents when a person is confronted with railings,
open edges, etc.--anything that might conceivably permit him to fall. Open
glass walls or floors, by extension, can trigger the fear also. But sitting in
a seat with seat belt attached and looking out a small window that one can
obviously not fit through isn't likely to trigger a fear of falling or
heights.

(who feels secure inside an airplane?)

Pilots and experienced passengers.

You can feel 'fear of falling' laying on a floor!

That would be very unusual.

Do you know where the 'fear of falling' comes from?

A fear of falling is wired in. All normal people have some degree of fear of
falling. Even babies automatically fear falling and heights. Only when it
becomes very extreme is it considered pathological and a phobia.

Fear of falling/heights can be aggravated by vestibular problems that affect
the ability to balance for walking and standing. Older people with such
problems often develop a strong fear of heights and falling (and justifiably
so, to some extent). Both fears tend to get worse with age in most people.
Women are more often affected than men.