Hello Marc-Dirven,

Would you check this please :

3-bar menu (or Tools) => Options => General Under "Applications" => Content Type, scroll down to Portable Document Format (PDF) - under "Action" did you choose "Preview in Firefox"  ?

Next, would you please take a look at this article :

https://techdows.com/2015/08/how-to-change-the-default-pdf-viewer-in-windows-10.html

McCoy said

Hello Marc-Dirven, Would you check this please : 3-bar menu (or Tools) => Options => General Under "Applications" => Content Type, scroll down to Portable Document Format (PDF) - under "Action" did you choose "Preview in Firefox"  ? Hi, it is set to "Preview in Firefox", and when I try to change it in Windows 10 as the link suggested, it pop backs to Edge. Also, if I change it to for e.g. Adobe, it doesn't pop back to Edge, so I'm thinking this has to do with Firefox. Next, would you please take a look at this article : https://techdows.com/2015/08/how-to-change-the-default-pdf-viewer-in-windows-10.html

The "Preview in Firefox" thing didn't work and was already set this way. Also when I try to change it to Firefox in Windows 10, it pops back to Edge. But if I change it to for e.g. Adobe, it doesn't pop back to Edge, so I'm thinking this has to do something with Firefox.

If changing the PDF viewer in Windows (as described in the techdows article) doesn't work, then maybe you'd like to post your question on the offcial Microsoft support forum :

https://support.microsoft.com/nl-nl

(As you appear to be Dutch, I've chosen the Dutch branch)

A new error occured. When I left click the file and select open with Firefox (this worked fine previously) an infinite number of Firefox internet tabs opens, all loading the PDF.

Marc-Dirven said

A new error occured. When I left click the file and select open with Firefox (this worked fine previously) an infinite number of Firefox internet tabs opens, all loading the PDF.

Sounds like the same problem in this thread :

https://support.mozilla.org/en-US/questions/1194929