Bathmophobia (from Common Phobias):
Bathmophobia is the fear of stairs or steep slopes. The origin of the word bathmo is Greek (meaning a step) and phobia is Greek (meaning fear).
Basophobia (from Wikipedia):
The fear of falling (FOF), also referred to as basophobia (or basiphobia), is a natural fear and is typical of most humans and mammals, in varying degrees of extremity.