The cable sheathing will soften and melt in a fire - so if you want to seal around the cables you will need something that will expand to fill the gaps. A big factor is the depth of the seal - much easier to get an effective seal in a thick wall than a thin one, for example.
Intumescent pillows are OK if the seal is deep and there are no big gaps between the cables, but they don't expand a lot, so they're not much good if you've got a whopping great pile of 50mm cables. The only thing that I know of that would have a chance, in that case, would be one of the high-expansion systems, such a Sealmaster's 'Firefoam'; Intumescent Seals' 'Therm-A-Foam', or Environmental Seals might have a product that would do it (any company that sells an intumescent cat flap obviously has an apetite for the weird and wonderful). The key is to demand that they can assure you that their products would work, in the situation you have. If it's a very demanding application, they might need to do some fire testing to back this up.
I'd steer away from the high-expansion foams that are applied from an aerosol can - they often have claims for high fire resistance, but if you investigate they've usually only been tested sealing relatively small holes in thick, inert walls. I'm not aware of any that have solid evidence that they work where cables transit walls or floors.
