why are halogens considered deactivating groups while methoxyl groups are activating?
Because fluorine has such a high electronegativity, its inductive withdrawing effect is much larger than any resonance electron-donating effect.

The other halogens have poor orbital overlap with the pi system of an aromatic ring, which precludes any appreciable resonance electron-donation (this is why you almost never see a C-X double bond). Therefore, their inductive capabilities lead to a net deactivation of the aromatic ring.

This is why an alkoxy group is more activating  than a halogen. Oxygen does have a higher electronegativity than does chlorine, bromine or iodine. However, oxygen's orbital overlap with carbon allows it to more readily donate into the ring's pi system. The larger halogens aren't able to do this (to an appreciable extent).