According to Bulbapedia, critical hits ignore halved Attack state from Burn in Generations I and II but not in Generations III-VI. (
http://bulbapedia.bulbagarden.net/wiki/Critical_hit)
I have experienced something contrary, however. As seen on an attached replay (
http://replay.pokemonshowdown.com/lc-212011330), in turn 3 Ferroseed is burned before it uses Bullet Seed. 4 hits from Bullet Seed deal damage equivalent to 8% of its target's maximum HP, whereas a critical hit deals damage equivalent to 24% of its target's maximum HP. As the damage was tripled and critical hit multiplies the damage by 1.5, the damage was doubled by another factor which seems to be ignoring the Burn, which should not take place.