Question

BF3 is less acidic than BCl3 although fluorine is more electronegative than chlorine

Answer 1

Lewis acids are electron acceptors and boron halides have six electrons in its outermost orbit and Boron is electron deficient.

Back bonding between fluorine 2p and empty 2p orbitals of boron occurs more effectively than for other halides which require 3p, 4p orbitals of halogen.
Thus, the acidity of BF₃ is less than other halides.