Userscript loads but will not run

I asked blackbox.ai to generate a usercript that would create a polyfill for Array.prototype.findLast.
This method is defined in modern javascript, but may be missing on older browsers.
I tested with an old browser.
I can see the console.log messages, which means the script is executed.
Nevertheless, when I execute Array.prototype.findLast in the console it says undefined.

Here's the userscript:

// ==UserScript==
// @name         Array.prototype.findLast Polyfill
// @author       ychaouche
// @namespace    ychaouche.scripts
// @version      2024-11-04
// @description  Add findLast method to Array.prototype if it doesn't exist
// @match        *://*/*
// ==/UserScript==

(function() {
    'use strict';

    if (!Array.prototype.findLast) {
        Array.prototype.findLast = function(callback, thisArg) {
            if (this == null) {
                throw new TypeError('Array.prototype.findLast called on null or undefined');
            }
            if (typeof callback !== 'function') {
                throw new TypeError('callback must be a function');
            }

            const list = Object(this);
            const length = list.length >>> 0;

            for (let i = length - 1; i >= 0; i--) {
                if (i in list) {
                    const element = list[i];
                    if (callback.call(thisArg, element, i, list)) {
                        return element;
                    }
                }
            }
            return undefined;
        };

        console.log('Array.prototype.findLast polyfill has been added.');
    } else {
        console.log('Array.prototype.findLast already exists.');
    }

    // Optional: Test the polyfill
    var testArray = [1, 2, 3, 4, 5];
    var lastEven = testArray.findLast(num => num % 2 === 0);
    console.log('Last even number:', lastEven);
})();

Here's the console trace showing the console.log messages, as well as an undefined when I try to access Array.prototype.findLast myself after the page loads:

Array.prototype.findLast polyfill has been added. Array.prototype.findLast-Polyfill.user.js:37:17
Last even number: 4

>> Array.prototype.findLast
undefined

The problem wasn't the script itself,
it was the CSP policy of the website I was visiting.
The script worked well on another website.
I am using firefox,
and it seems you can't disable CSP for a particular website or tab in firefox.