Hi Larry, Firefox has a built-in viewer. If you check the "Portable Document Format" setting on the Options page, it's the one called "Preview in Firefox". This article shows how to check on that: [[View PDF files in Firefox]]. Does that work for this sample PDF: https://jeffersonscher.com/res/Mixed_Content_Blocking.pdf Websites can instruct Firefox to download a PDF instead of showing it in a tab, or websites can provide only general information about the file rather than using the official "application/pdf" content type. If the site does that, Firefox will not open the PDF in a tab, so you definitely can get inconsistent results.