Hopefully there will be a checkbox for whether the Downloads list opens automatically in a future update. For now, you can use a hidden setting to turn it off:
(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
More info on about:config: Configuration Editor for Firefox. Please keep in mind that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.
(2) In the search box in the page, type or paste browser.download.alwaysOpenPanel and pause while the list is filtered
(3) Double-click the preference to switch the value from true (open it) to false (do not open it)
