Hello Guillermo220,
thank you for your support. I tried to download a file, and the sandbox, in this case, seems to work fine. I am not a security expert, so I do not have a real hypothesis on what could go wrong if the sandbox does not work properly. Since the options that I selected were made to adapt the level of security to one's needs, it bothers me that they do not work as expected because I don't know what other functions could be bugged and allow a threat to escape the sandbox.
As explained before, though, the sandbox works fine until I launch Firefox in it. Then it stops working correctly and all other browsers are affected. Have you tried reinstalling Avast to confirm or deny this behavior?
Thank you again.