In Firefox open the Firefox Menu.
Click on “Preferences”, then select the “Advanced” tab.
Select the “Certificates" tab and click “View Certificates”.
Select the “Authorities” tab.
From the list of Certificate Names, scroll down and select the "PortSwigger CA" entry in the list (this is a sub-entry under PortSwigger). The entry should now be highlighted.
Click "Delete or Distrust".
Click "OK" in the confirmation dialog.
Confirm that the "PortSwigger CA" entry has been removed.