Firefox implements new W3C File API specification
Web applications can access local files and upload multiple files.
The current fourth beta of Firefox 3.6 comes with support for the File API. The relevant W3C draft specification is from Mozilla developer Arun Ranganathan. The File API provides different objects. This includes FileList containing an array of individually responsive files. It should be integrated over an input tag in HTML 5 pages. Several files can be uploaded at once.
This allows Firefox to upload via drag and drop, so that files can be moved easily from the desktop into a corresponding area in a website. It is also possible to directly create smaller thumbnails in the browser and display, even during the upload of files running in the background.
Checkout developer.mozilla.org for examples for using the File API.