In Gen 1 only, using Haze could cure the opponent of being frozen.
In Gen 1-2, all Fire type moves that could cause a burn (Every fire type move, excluding Fire Spin) can thaw out a frozen Pokemon.
In Gen 2 only, when used on a frozen Pokemon, Tri Attack had a 1/3 chance of thawing them.
From Gen 3 onward, all Fire type moves that could cause damage can cause a frozen Pokemon to thaw, excluding Fire type Hidden Power.
As of Gen 4, Pokemon can now be thawed from being hit by a Fire type Hidden Power.
As of Gen 6, Pokemon can be thawed from being hit by Scald or Steam Eruption. However, if they do not take damage from either of the moves, they will not be thawed.
Also, the moves Flame Wheel, Flare Blitz, Fusion Flare, Sacred Fire, Scald, and Steam Eruption will cause the Pokemon using it to thaw if they are frozen. Burn Up can also do this, if the user is Fire type. On top of that, Heal Bell will cause any Pokemon in the user's party to be cured of being frozen.