Originally Posted by Baraka_Guru

Firefox will calculate the average time based on the file size and the average download speed as a real-time calculation. If a server gets busy, the download speed will drop and so Firefox will recalculate the time based on the new speed. The time calculation is only really accurate when the download speed is consistent. Either way, it's only an estimate.

I think it depends on the server. If either Firefox or your ISP severs the connection to the download server (whether by closing the program or dropping the Internet connection), the server may or may not allow you to resume the download. It's a setting made possible on the other end.