Open Mozilla Firefox and type about:config in the address bar and press Enter. It'll show you a warning message, click on "Accept the Risk and Continue" button. It'll open Firefox's hidden secret advanced configuration page i.e. about:config page.

To enable and always show "Downloads" button in toolbar, double-click on browser.download.auto-hide Button preference and set it to false. Alternatively, you can click on the Toggle icon given next to the preference name.