code: function findFirefox() {
    if (foundBinary) {
      return foundBinary;
    }
  
    if (process.platform === 'darwin') {
      var osxExe =  '/Applications/Firefox.app/Contents/MacOS/firefox-bin';
      foundBinary = io.exists(osxExe).then(function(exists) {
        return exists ? osxExe : null;
      });
    } else if (process.platform === 'win32') {
      foundBinary = defaultWindowsLocation();
    } else {
      foundBinary = promise.fulfilled(io.findInPath('firefox'));
    }
  
    return foundBinary = foundBinary.then(function(found) {
      if (found) {
        return found;
      }
      throw Error('Could not locate Firefox on the current system');
    });
  }
docs: Locates the Firefox binary for the current system.
  @return {!promise.Promise.<string>} A promise for the located binary. The
  promise will be rejected if Firefox cannot be located.