Are you sure it is the [[View PDF files in Firefox or choose another viewer#w_use-firefoxs-built-in-pdf-viewer|Firefox PDF viewer]]? Some websites use their own PDF viewer. Does it still happen in [[Diagnose Firefox issues using Troubleshoot Mode|Troubleshoot Mode]]?

Yes, it is the Firefox PDF viewer and yes, it still happens in troubleshooting mode.

Are all PDFs affected or only some? Can you link an example PDF? Does it still happen in a [[Profile Manager - Create, remove or switch Firefox profiles|new profile]]? An easy way to test a new profile is to install [https://aurora.mozilla.org Developer Edition] and see if it happens there or [[Refresh Firefox - reset add-ons and settings|refresh your existing profile]]. Make sure you stay signed out of your sync account when testing.