please use `OS.Path.join()` for other OS (you know, I mean Windows...), you can create a wrapper in utils.js for node.js.

```javascript
Cu.import('resource://gre/modules/osfile.jsm');
var fullPath = OS.Path.join('firstDir', 'secondDir');
```

please reference https://developer.mozilla.org/en-US/docs/JavaScript_OS.File/OS.Path