Firefox 20+: re-enable old download manager

[Gingerbread Man]

Starting in Firefox 20, downloads are displayed in a pop-up panel attached to an icon on the navigation toolbar. For details, see the following article.
https://support.mozilla.org/kb/find-and-manage-downloaded-files

If you'd like to disable this feature in favor of the old Downloads window, perform the following steps.

1. Type about:config in the address bar and press Enter.
2. Press the big button to bypass the warning.
3. In the search box, paste browser.download.useToolkitUI
4. In the search results, double-click browser.download.useToolkitUI to set its value to true.