See [[/questions/761494]] * Move the old .part partial download and the final file without the .part that has size 0 to another location. * Start a new download and pause it, don't close Firefox. * Copy the two files (.part and 0 byte final) that you moved above back to the download location to replace the new files that were created. * Resume the download in Firefox. If that doesn't work then the server may not support resuming.