Is the builtin PDF Viewer still enabled if you check pdfjs prefs (pdfjs.disabled) on the about:config page ?

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

See also:

• https://support.mozilla.org/en-US/kb/view-pdf-files-firefox
• https://support.mozilla.org/en-US/kb/disable-built-pdf-viewer-and-use-another-viewer

pdfjs.disabled is set to its default value of false. That's not it.

I think I figured it out. The website's links to the pdf files were ftp. When I tried a website with https it worked as expected.

How does the server send the PDF file if you check this in the Web Console (Requests and XHR) and Network Monitor ?

• "3-bar" menu button or Tools -> Web Developer
• https://developer.mozilla.org/en-US/Tools/Web_Console

If the file is send a "application/pdf" then Firefox will open the file in a Firefox tab. If the file is send with another content type then Firefox will offer to save the file.

I am unable to determine how the file is sent. Looking at the source, there's a <a> tag with the href="ftp://..." but also has target="_blank" so it's opening a new tab and then doing the save box. So there is no console view on the new tab.

I'm not going to worry about it any more.

You can alternatively use the Browser Console and enable messages from content processes via the cogwheel button in the Console.

• https://developer.mozilla.org/en-US/Tools/Browser_Console