I know it makes fire moves less effective, but by how much? Does it make fire neutral? Does it make it one stage less effective? Does it make fire always not very effective? Please help.
The fireproof ability cuts all fire damage taken in half, no matter if it's super effective, neutrally effective, or not very effective. In effect, it makes super effective moves (2x) neutrally effective, neutrally effective moves not very effective, and so on. It also halves damage taken from burns.
Source: https://m.bulbapedia.bulbagarden.net/wiki/heatproof_ability
Like you imply it does not necessarily make the move that should be super effective 'not very effective', but it just lowers the damage done by the Fire-type move (in that case super effective would do the same damage as effective) or the burn status by half.
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