Hi weenusman, you are not completely alone in this problem. We had a similar report last week.

(1) Test on multiple sites.

Some sites may not use the exact content type application/pdf that Firefox uses to recognize Portable Document Format (PDF) files.

(2) Try switching among the options to see whether that helps.

This article will get you to that part of the Options page: View PDF files using Firefox’s built-in viewer. Like "turn it off and on again" switching to Adobe and then back to Firefox may help the message get through.

If that doesn't help, check out the other thread I mentioned to see whether that more drastic remedy helps: Cannot view PDF in firefox.

In case this comes up again, I created an extension that can help with this problem. I suggest leaving it off until you run into a problem, and only turning it on (using its toolbar button) while you are downloading files that need it. Otherwise, because it intercepts every response from every web server, it may slow down your browsing.

https://addons.mozilla.org/firefox/addon/content-type-fixer/

If you try it, let me know how it goes.