download-n-upvote.js

// ==UserScript==
// @name         Download & Upvote
// @version      0.3
// @description  Disclaimer: my js is kinda rusty, feel free to comment if you spot a derp
// @author       DeliciousBlackInk
// @include      /^https:\/\/(derpibooru\.org|trixiebooru\.org|derpiboo\.ru)\/(\d)(.*)$/
// @grant        none
// ==/UserScript==

var downloadButton = document.querySelector('[title^="Download this image"]');
downloadButton.addEventListener('click', upvote, true);

/* the pop-up download location window Firefox throws at us 
blocks the site's script. here's a quick & dirty hack: remove the
download link from the button and open it later, once the image's been
upvoted */
var downloadLink = downloadButton.getAttribute('href');
downloadButton.removeAttribute('href');
/* make the empty button seem as an ordinary link */
downloadButton.style.cursor = 'pointer';

function upvote() {
    var upvoteButton = document.querySelector('.interaction--upvote');
    if (!upvoteButton.classList.contains('active')) {
      upvoteButton.click();
    }
    /* here's the point of that monstrous hack: force the link to open in another tab */
    window.open(downloadLink, '_blank');
}