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 and they may have been removed by anti-virus or cleanup software.

• http://kb.mozillazine.org/Unable_to_save_or_download_files

Thanks for the answer, Cor-el. I was aware of this but since I didn't find the un-openable downloads in my antivirus quarantined files, I was wondering if they might be somewhere else. I don't have cleanup software, (to the best of my knowledge). Also it's strange that all of my downloads are always available right after download, (that is, the open button grays out only if the file is older than a day or two, and also seems to happen if I had previosly opened the same download (as if you can open a download only once).