docs: function defaultWindowsLocation() {
    var files = [
      process.env['PROGRAMFILES'] || 'C:\\Program Files',
      process.env['PROGRAMFILES(X86)'] || 'C:\\Program Files (x86)'
    ].map(function(prefix) {
      return path.join(prefix, 'Mozilla Firefox\\firefox.exe');
    });
    return io.exists(files[0]).then(function(exists) {
      return exists ? files[0] : io.exists(files[1]).then(function(exists) {
        return exists ? files[1] : null;
      });
    });
  }