Firefox can't open files anymore

CFP3 asked me about which predefined security policy to apply after I clicked on a link to a file (XLS, PDF etc.) using Firefox.
I probably chose the wrong answer and as a result I must now first save the file and then open it with the associated application.
If I choose Open with (instead of Save to disk) I get a Download Error message stating that the file (located in a Temp path) could not be opened.
How to revert to the prior state?

Incidentally, IE continues to work as usual.

Delete the rules for FF you made. In the main screen go to Defense + → Advanced → Computer securituy policy. Now delete the rules for FF.

Do the same thing for the network rules when needed.In the main screen go to Network → Advanced → network security policy. Now delete the FF rules.

Thanks for the clue.