Firefox by default can't open a pdf you have to with my understanding of pdf need to have a addon installed to it can open it. And or have Acrobat reader installed so it can open a pdf file or similar. I could be wrong but that is my general understanding of it.

Firefox should be able to open locally saved PDF files or set as the default application for PDF files. Firefox used to set itself automatically as the default PDF viewer when you install Firefox and detects that there is currently no application set. Otherwise you should be able to do this in Windows File Explorer via "Open with".

• Bug 746102 - Add .pdf as an understood file association for Firefox
• https://support.mozilla.org/en-US/kb/view-pdf-files-firefox

The pdf can be opened with Firefox in different ways. There is no problem with that. The problem is that Windows doesn't allow me to set Firefox as the default app to open pdf's. I tryed different ways. When I try for instance to set the default app by going to the settings of a pdf-file and try to change the Open with - Change there, I get a list of apps with which I can open the pdf in the future. Firefox is listed there but when I choose Firefox there and press OK afterwards, Firefox isn't being choosen. The Open with field remains the app from before

Does modifying HKEY_CLASSES_ROOT\.pdf directly with the Windows Registry work?

No it doesn't. I exported the corresponding registry key on an other pc (on which everything works ok) and imported it in the registry of my pc. No change. Apparently the problem seems to be linked to my pc only, so something in the configuration must be damaged. Sadly! Will use Chrome, that works. Will be fixed in some distant future, when I decide to reinstall Windows, if ever. Thanks for helping.