I made a Global Loot Modifier that doesn't seem to be working. I've probably messed up in the FireSwordInChest.java file. Here is FireSwordInChest.java:

package com.juder.elementalswords.event.loot;

import com.google.gson.JsonObject;
import net.minecraft.resources.ResourceLocation;
import net.minecraft.util.GsonHelper;
import net.minecraft.world.item.Item;
import net.minecraft.world.item.ItemStack;
import net.minecraft.world.level.storage.loot.LootContext;
import net.minecraft.world.level.storage.loot.predicates.LootItemCondition;
import net.minecraftforge.common.loot.GlobalLootModifierSerializer;
import net.minecraftforge.common.loot.LootModifier;
import net.minecraftforge.registries.ForgeRegistries;

import javax.annotation.Nonnull;
import java.util.List;

public class FireSwordFromChestAdditionModifier extends LootModifier {
    private final Item addition;

    protected FireSwordFromChestAdditionModifier(LootItemCondition[] conditionsIn, Item addition) {
        super(conditionsIn);
        this.addition = addition;
    }

    @Nonnull
    @Override
    protected List<ItemStack> doApply(List<ItemStack> generatedLoot, LootContext context) {
        System.out.println("YO!");
        generatedLoot.add(new ItemStack(addition, 1));
        return generatedLoot;
    }

    public static class Serializer extends GlobalLootModifierSerializer<FireSwordFromChestAdditionModifier> {

        @Override
        public FireSwordFromChestAdditionModifier read(ResourceLocation name, JsonObject object, LootItemCondition[] conditionsIn) {
            Item addition = ForgeRegistries.ITEMS.getValue(new ResourceLocation(GsonHelper.getAsString(object, "addition")));
            return new FireSwordFromChestAdditionModifier(conditionsIn, addition);
        }

        @Override
        public JsonObject write(FireSwordFromChestAdditionModifier instance) {
            JsonObject json = makeConditions(instance.conditions);
            json.addProperty("addition", ForgeRegistries.ITEMS.getKey(instance.addition).toString());
            return json;
        }
    }
}

Any help would be appreciated.

Nope, that class looks fine.
Did you register the serializer? Did you add it to the forge/global_loot_modifiers.json list?

If you are using 1.19 the GLM doesn't use a serializer anymore.

It uses Codecs.

You can see some examples in the forge tests:

https://github.com/MinecraftForge/MinecraftForge/blob/1.19.x/src/test/java/net/minecraftforge/debug/gameplay/loot/GlobalLootModifiersTest.java

I am using 1.18.2.

I did not register the serializer. Thanks.