intervene does not automatically trust certificates for Firefox. Firefox uses its own trusted certificate store and does not rely on the operating system certificates, which makes it much harder to do this automatically.
It may be possible to instruct Firefox to trust a new certificate automatically, so this may change in the future.
When you visit a URL with
intervene running with Firefox, you'll first be prompted that the certificate is not trusted, click "Advanced" and then "Add exception" to add the certificate to the trusted list. The certificates are only valid for 7 days, so if you run
intervene again after 7 days, you'll be prompted again.