A likely cause is that the servers sends the file as text/plain. You can check that in Tools &gt; Pa

A likely cause is that the servers sends the file as text/plain.
You can check that in Tools > Page Info > General when the file opens in a new tab.
Firefox will open the file in a tab in this case and your only option is to right click and use save link as to save the file or save the page once opened in a new tab/window.