If Open and Open Containing Folder in the right-click context menu in the Download Manager window are grayed then that means that the file is no longer in the original download location.<br /> It is possible that your anti-virus software is corrupting the downloaded files or otherwise interfering with downloading files by Firefox.<br /> Try to disable the real-time (live) scanning of files in your anti-virus software temporarily to see if that makes downloading work.<br /> See: * http://kb.mozillazine.org/Unable_to_save_or_download_files

Thank you for your solution, it seems to be working just fine.