This add-on has been developed over four years ago and remains pertinent IMO.
I'd love to have it updated (though it runs fine here & now on Firefox 33.1.1) together with the availability of a menu drop-down toolbar button : that would be the ultimate as far as I'm concerned.
Quick access to files/folders from within the OS is always possible, nevertheless having a link from the browser itself is sometimes, for often called files, in situations like full-screen mode, a real plus. This add-on does it, but the toolbar it offers is cumbersome in terms of screen space and the alternative to call it from Firefox's menu is not handy. Just a drop-down toolbar button, I insist :)