I know previously when I clicked on PDFs they would be downloaded. I'm assuming I changed something either within Firefox directly or via an app - but I can't find anything. Now when I click a download link a new tab starts to open - then clears and returns to the original page. Nothing is ever downloaded. The only way I can is by pressing and holding until a "copy link" appears (assuming the page allows it) and pasting in a download app.

How can I set the downloaded or associated apps?