Firdoch Core's last ability overwrites all previous effects that set its base power and toughness to specific values. Any power- or toughness-setting effects that start to apply after the ability resolves will overwrite this effect.

Effects that modify a creature's power and/or toughness, such as the effect of Appeal to Eirdu, will apply to the creature no matter when they started to take effect. The same is true for any counters that change its power and/or toughness and effects that switch its power and toughness.