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