When pursuing their goal, mobs do not use the fire resistance effect, although they have the opportunity to do so.

If piglin sees a desiccant skeleton that is his enemy (in the game's code), he will immediately go at him to take his life. But if the piglin does not have access to the path to the desiccant skeleton or it is blocked (for example, by lava), the piglin will notice the desiccant skeleton and also pay attention to it, but of course he will not go into the lava just to deal with the desiccant skeleton. And piglin will not do this even if it is affected by the fire resistance effect.

How to reproduce:

Create points A and B, where there will be a pigline at point A, and a desiccant skeleton at point B.

Block the path from point A to point B, in other words, make piglin angry at the desiccant skeleton, but unable to harm it. This should be done using ordinary lava.

At point A, create a piglin by running the command: /summon minecraft:piglin ~ ~ ~ {IsImmuneToZombification:1}

At point B, create a desiccant skeleton by running the command: /summon minecraft:wither_skeleton ~ ~ ~ {NoAI:1}

Observe that piglin became alarmed at the sight of the desiccant skeleton and began to make angry noises.

Give it the fire resistance effect forever by executing the command: /effect give @e[type=minecraft:piglin] minecraft:fire_resistance 1000000.

Please note that piglin still does not try to contact the lava to deal with the desiccant skeleton, apparently in the game code mobs do not know how to understand that if they are affected by the fire resistance effect, they can safely dive into the lava in order to deal with their enemy.