At least in the Pen and Paper game, a Frostbrand sword is +3 all the time, and gets +6 vs creatures of fire and/or heat (Hellhounds, red dragons, fire elementals, etc.) or any creature that lives or dwells in fire. It gives protection vs. fire as a Ring of Fire Resistance and can extinguish any fire into which it is thrust (including Wall of Fire, the spell, but not spells that create fire only momentarily, such as Fireball, Flamestrike, Burning Hands and so on).
I haven't read the Drizzt books but to accurately solve this issue of naming the weapons, I'd say look at the timeframe of the game and the books, not at 2nd vs 3rd edition. I have heard the in one of the later books the sword is given the name Icingdeath. Compare the year BG is set to whenever this takes place in the Drizzt Saga and we can determine if it should or shouldn't be referred to as Icingdeath. Twinkle on the other hand, I feel should be named ingame as such.
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ON EDIT: I looked it up and I'm wrong. Drizzt was around during 1E.
Anyway, a defender sword in AD&D is a +5 weapon, and every round, you can divide up that 5 between to hit/damage and AC.
This would be extremely cumbersome to put into Baldurs Gate, so I think he developers just decided on a +3/+2 split.