index.js

var self = require('sdk/self');
var child_process = require('sdk/system/child_process');
var buttons = require('sdk/ui/button/action');
var tabs = require('sdk/tabs');
var settings = require('sdk/simple-prefs');
var contextMenu = require('sdk/context-menu');

var button = buttons.ActionButton({
  id: 'mpv-link',
  label: 'Open with mpv',
  icon: {
    "16" : "./mpv-16.png",
    "32" : "./mpv-32.png",
    "64" : "./mpv-64.png"
  },
  onClick: handleClick
});

function handleClick(state){
  console.log(tabs.activeTab.url);
  var mpvExec = settings.prefs.mpvPath;
  child_process.spawn(mpvExec, [tabs.activeTab.url]);
}

function handleContextMenu(link) {
  console.log(link);
  var mpvExec = settings.prefs.mpvPath;
  child_process.spawn(mpvExec, [link]);
}

var menuItem = contextMenu.Item({
  label: 'Open link with mpv',
  image: self.data.url('mpv-16.png'),
  context: contextMenu.SelectorContext('a[href]'),
  data: settings.prefs.mpvPath,
  contentScript: 'self.on("click", function(node, data){ self.postMessage(node.href); })',
  onMessage: handleContextMenu
});