Firefox does not show me if a download has been interrupted. On the downloads tab it just shows an estimated time; in the library it shows a more detailed status bar, but it doesn't show when a download has been interrupted, it just shows how much has been downloaded, and a transfer rate that seems to be the last recorded, rather than the current rate of zero. (I have a separate bandwidth monitor which shows total upload and download rates on my computer, this is how I know the download has stopped)