I can see benefits to keying off the IP, but also to keying off some cookie that can expire shortly. One of the reasons I imagine a download might fail could be because of a spotty of problematic VPN or proxy, or attempting to get it from a location that can't handle it well if somewhat large (some random coffee shop wifi that's overused).

There's probably enough complexity and possibility for abuse in allowing automated requests for files again (i.e. a button on the view page) or special logic for second attempts that the safest option is just to have the receiving party ask for the file again through whatever medium originally kicked off the request (an email, an IM, etc).

Firefox could do any number of things to make it easier on the user, but I expect them to take my security and privacy very seriously and to error on the side of those ideas rather than usability, so hopefully if they come out with something it's not at odds with those goals.