I have changed to manual proxy, 127.0.0.1 on port 8080. That got me through to the certificates at http://mitm.it/ which I have downloaded and installed. But now firefox (latest version 89.0) is rejecting every web page because of mitmproxy. Under the advanced tab of the error page is this explanation:
Firefox uses the Mozilla CA store to verify that a connection is secure, rather than certificates supplied by the user’s operating system. So, if an antivirus program or a network is intercepting a connection with a security certificate issued by a CA that is not in the Mozilla CA store, the connection is considered unsafe.
Is there a workaround for this?