A creature with fire immunity never takes fire damage. It has vulnerability to cold, which means it takes half again as much (+50%) damage as normal from cold, regardless of whether a saving throw is allowed, or if the save is a success or failure.
Find topic in: Epic, Rules of the Game
dungeons d&d Immunity Fire Special Conditions wizards d&d Abilities Immunity Conditions srd dungeons 3.5 Special dnd dungeons Conditions dragons & wizards 3.5 & & d&d d&d dragons d&d Conditions & d&d rpg d20 Fire wizards dragons rpg dungeons Conditions Conditions