I have this Firefox extension that someone made for me. It prevents websites from opening download boxes and redirects and stuff like that. It works very well. It will prevent some websites from opening completely or working properly, but for my needs it works great.

Can this be turned into a Greasemonkey script, or it can only work as an extension?

main.js

    'use strict';

const mimeTypeMatch = function (header) {
  if (header.name === undefined) {
    return false;
  }
  if (header.name.toLowerCase() != 'content-type') {
    return false;
  }
  if (header.value.startsWith('image')) return false;
  if (header.value.startsWith('text')) return false;
  if (header.value.startsWith('font')) return false;
  if (header.value.startsWith('application/json')) return false;
  if (header.value.startsWith('application/javascript')) return false;

  console.log('cancel content type');
  console.log(header);
  return true;
};

const headersReceived = function (request) {
  return new Promise((resolve) => {
    let filter = false;

if ( (['somesite.com']).find( u => {
  const uri = new URL(request.originUrl);
  return u === uri.hostname;
}) ) {
  resolve({});
  return;
}

    request.responseHeaders.forEach(h => {
      if (mimeTypeMatch(h)) {
        filter = true;
        return;
      }
      if (h.name.toLowerCase() === 'content-disposition') {
        filter = true;
        return;
      }
    });

    if (filter) {
      resolve({
        cancel:true
      });
      return;
    }

    resolve();
  });
};

browser.webRequest.onHeadersReceived.addListener(
  headersReceived,
  {
    urls: ['<all_urls>']
  },
  [
    'blocking',
    'responseHeaders'
  ]
);

manifest.json

    {
  "manifest_version": 2,
  "name": "mime-type-cancel",
  "version": "1.0",
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "<all_urls>"
  ],
  "background": {
    "scripts": [
      "main.js"
    ]
  }
}

Thanks