instagramDownloader.user.js

// ==UserScript==
// @name         Instagram Downloader
// @namespace    http://stripedpurple.io
// @version      2018.12.29.2
// @description  Download images from instagram. This was created so that I can test lightroom presets and LUTs on existing photos.
// @author       Austin Barrett
// @match        *://www.instagram.com/*
// @grant        none
// ==/UserScript==

(function () {
    'use strict';
    
    var newStyle = document.createElement('style');
    newStyle.innerHTML = '._9AhH0 { pointer-events: none;}';
    document.head.appendChild(newStyle);

    var oldDownloadLink = "";
    function downloader() {
        var imgElem = document.querySelectorAll('article.M9sTE.L_LMM  .FFVAD')[0];
        var downloadLink = imgElem.currentSrc;

        imgElem.srcset.split(',').forEach(function (d) {
            var e = d.split(' ');
            if (e[1] == '1080w') {
                downloadLink = e[0];
            }
        });

        if (oldDownloadLink !== downloadLink) {
            var oldElem = document.getElementById('downloaderBtnFTW')
            if (oldElem !== null) {
                oldElem.parentNode.removeChild(oldElem);
            }

            var downloadBtn = document.createElement('span');
            downloadBtn.id = 'downloaderBtnFTW';
            downloadBtn.innerHTML = '<button class="dCJp8 afkep _0mzm-"><a href="' + downloadLink + '" download target="_blank"><img width="32px" src="https://i.imgur.com/qyt2rL0.png"></a></button>';
            document.querySelectorAll('section.ltpMr.Slqrh')[0].appendChild(downloadBtn);
            oldDownloadLink = downloadLink;
        }
    }
    setInterval(downloader, 500)
})();

Под Firefox 85.0b4 (64-bit) и Tampermonkey 4.11.6120 - работает, под Firefox 84.0.1 x64 - нет.