Downloads not visible in Download Manager
From MozillaZine Knowledge Base
Downloads window is empty
If downloads currently in progress do not appear in the Downloads window:
Note: Firefox 3 stores download information in the downloads.sqlite file instead or downloads.rdf. Because of Firefox 3 Download Manager improvements  there should be little need to delete downloads.sqlite, except in cases of file corruption.  If you do delete the downloads.sqlite file, make sure to also delete downloads.rdf, if it exists.
If completed downloads do not appear in the Downloads window, check your Privacy settings to make sure you haven't disabled the download history.  (Firefox 2 and above) Click "Tools -> Options -> Privacy" to open the Privacy panel.
Downloads window does not open or stay open
If the Downloads window does not automatically open when you start downloading a file, make sure Firefox is set to show the downloads window. In Firefox 2 and above:
The Downloads window will close immediately when the download completes (starting in Firefox 4?), or not at all if the download is very fast. Use "Ctrl+J" to display downloads window at any time. To restore old behavior: