TL;DR;
Fire Giants (should not be) / (are not) immune to mind spells anywhere in the world but layonara (if they are immune to them here)
Fire Giants in the OC are not immune to mind spells
They are immune to fire, but vulnerable to cold... they have a will save of 5... no immunity to mind spells, not on their skin, their weapon, or their armor.... not granted by a script, special ability, feat, or item.
there is one fire giant blueprint in the OC that has a will save of 8.
To see this you can open any of the OC modules by copying them from the nwm directory to the modules directory of your NWN install... rename all of them to have the extension .mod and then you can open them in the toolset and edit/look as much as your heart desires...
Chapter 1 does not have any fire giants in the custom pallete or the module
Chapter 1E does not have any fire giants in the custom pallete (didn't search the module)
Chapter 2 does not have any fire giants in the custom pallete (didn't search the module)
Chapter 2E does not have any fire giants in the custom pallete (didn't search the module)
Chapter 3 has fire giants in the custom pallete
Chapter 4 does not have any fire giants in the custom pallete (didn't search the module)
If fire giants are immune to mind spells on layo, it's the only place in the world they are immune to them
https://dnd-wiki.org/wiki/Fire_Giant_(3.5e_Monster)
https://www.dandwiki.com/wiki/SRD:Fire_Gianthttp://forum.nwnights.ru/uploads/rulebooks/Core_Books/Monster_Manual.pdfyou can search the module and look for a special case if you want using edit -> find instance and look for the tags, or the blueprint resref (required to place an instance in the module) of the creature you're looking for.