Mozilla has introduced panel based download manager with Firefox 13, but till Firefox 26 they’ve offered preference in about:config to restore old download manager UI that shows downloads in a separate window, now the code for it has been removed in version 26 and the preference ‘browser.download.useToolkitUI’ too- if you’re still seeing that preference, you won’t be after Resetting Firefox.
Don’t worry? If you still not used to new download manager and want to view downloads in a window you can with Downloads Window extension.
Restore Old Download manager in Firefox 26+
According to developer, this extension replaces the ‘Downloads’ menu items and key accelerators and creates a pre-Firefox 26 browser.download.useToolkitUI-style separate downloads window.
Install the extension (no restart required) and immediately downloads will appear in a separate window if you start any file download, you may still want to hide downloads icon on navigation toolbar by dragging and dropping it onto customize Toolbar dialog -Menu>Options>Toolbar Layout.