TenFourFox version is 45.7 (it did the same in 45.6)
Preferences are set as you suggest - in fact these settings have not changed.
I tried your pdf test link above and I get a blank browser window.
The thing is, it used to work flawlessly then recently it just stopped. That's when I replaced v45.6 with 45.7 and there was no change.
Your link above works but this one doesn't it gives a download confirmation badge no matter if use built-in PDF viewer is checked or not. Curiously when use-built-in PDF viewer is unchecked with your link the file is downloaded without confirmation.
That's because that link (degoogled:
generates a MIME type of application/octet-stream
which is just "generic binary." The in-browser PDF viewer only
opens for application/pdf and related MIME headers.
Having the browser open the PDF viewer for anything ending in
.pdf is actually problematic for a number of reasons I
won't get into here, but suffice it to say I don't consider that a