If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block content.

• make sure your extensions and filters are updated to the latest version
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems
• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

Clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Remove the Cookies" for websites that cause problems:

• Options/Preferences -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Options/Preferences -> Privacy & Security
  Cookies and Site Data -> Clear Data -> Cached Web Content: Clear

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems
• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

I found out what exactly is happening. The blank popup window appears and the PDF file that should show in the popup is downloaded automatically. How do I change that? I want to handle the file from the popup window, not after it is already downloaded. It works as it should in Waterfox.

I found out what exactly is happening. The blank popup window appears and the PDF file that should show in the popup is downloaded automatically. How do I change that?

Change your PDF handling setting to "Open in Firefox" and see whether that fixes it: View PDF files using Firefox’s built-in viewer.

OK. That explains much. Strange things were happening on Newspapers.com when I was downloading PDF clippings recently. I have Acrobat, so I disabled the Firefox PDF option and will check to see if that solves the problem. Also explains why things worked as before on Waterfox, as I don't think that browser has the PDF feature. I will test my change and see what happens. Thanks.

Open in Firefox does not allow the PDF to be viewed in the popup window. Open in Firefox requires a right click and select open in new tab. Waterfox has a "preview in Waterfox" that does what Firefox used to do. Please bring that selection back! For now I will use Waterfox.

After trying the other settings, I went back to view in Firefox and now the popup window shows the pdf file. I also made sure this site has permission for popup windows.