When I click the Downloads (down arrow) button in the AiOS toolbar, it displays a dialog that says:
No downloads for this session.
Show all downloads.
When I click the "Show all downloads" link, the normal downloads window gets opened. Not in the sidebar.
It works fine when I select Tools -> Downloads from the menus.
I use the latest version of Firefox 64-bit (51.0.1) on a fully-updated Windows 10 PC