Firefox can't open files anymore

Help for v3
1

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.

2

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.

3

Thanks for the clue.