I have read all the applicable "troubleshooting" articles, re-set Firefox to the defaults, deleted the cache, etc. Have not been able to open any downloaded file from any source (including the newest Java and Firefox 17.0) and cannot find the downloaded file again except from the Firefox tools/download window. Even when I see the file in the download window, neither the icon nor the filename is active for double-clicking. Right-clicking shows the "open" and "open folder" options but they are greyed out.
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.
It is possible that your anti-virus software is corrupting the downloaded files or otherwise interfering with downloading files by Firefox.
See "Disable virus scanning in Firefox preferences - Windows"