Question

why on moving from Be to B does the value of the first ionization energy drop?

Answer 1

Therefore the energy required to remove one electron from the p orbital of Boron (B) is lower than the one required to remove one electron from the s orbital (fully filled) of Beryllium (Be).

Example is, the first ionization energy of oxygen(O) is lower than that of nitrogen (N). This also applies to the other periods.