Prevent websites stealing keyboard shortcuts I use all the time... Modified from https://gist.github.com/rodneyrehm/5213304.

anti-keygrabber.user.js

// ==UserScript==
// @name anti key-grabber
// @description Prevent web apps from capturing and muting vital keyboard shortcuts
// @grant none
// @version 1.2
// ==/UserScript==
(function(){
var isMac = unsafeWindow.navigator.oscpu.toLowerCase().contains("mac os x");
unsafeWindow.document.addEventListener('keydown', function(e) {

//   // No modifiers. Block any usage.
//   switch (e.keycode) {
//     case 116: // F5
//       e.stopImmediatePropagation();
//       return;
//   }
  
  // Block keys with modifiers. Cmd for OSX, Ctrl for Linux/Windows.
  var modifier = isMac ? e.metaKey : e.ctrlKey;
  if (!modifier) {
    return;
  }
  
  switch (e.keyCode) {
    case 75: // K - search
    case 76: // L - focus awesome bar
    case 82: // R - reload tab
    case 84: // T - open tab
    case 87: // W - close window
      e.stopImmediatePropagation();
      return;
  }
  
  if (isMac) {
      switch (e.keyCode) {
      case 188: // , (comma) - open settings [mac]
        e.stopImmediatePropagation();
        return;
    }
  }
}, true);
})();