After trying to solve this issue for a while (with no success, of course ) I posted a help request in the official Mozilla forums.

I got yesterday a very interesting answer. I'll check it right now and I'll report here if it is working

Ok, just tested and it is working corretly.

The solution is the following:

• Open a new tab in firefox and type about:config
• search for the item ui.key.contentAccess. In my browser the default value was 5
• double click on it
• Change the value to 4
• Enjoy

To be more precise, the reason why I want to do this change is that pressing ALT+S is a keyboard shortcut to post a reply here in the forum. Applying the above method, now it is possible again to use the keyboard shortcut to post a reply without the History menu opening.