Firefox
How to install the mitmproxy certificate authority in Firefox:
1. If needed, copy the ~/.mitmproxy/mitmproxy-ca-cert.pem file to the target.
2: Open preferences, click on "Advanced", then select"Encryption":

3: Click "View Certificates", "Import", and select the certificate file:

4: Tick "Trust this CS to identify web sites", and click "Ok":

You should now see the mitmproxy certificate listed in the Authorities tab.