Install Burp's CA Certificate In Firefox

1. In Burp Suite, go to Proxy > Intercept tab and disable intercept mode by clicking on the "Intercept is on" button.

In Firefox, navigate to a secure website, e.g., https://www.example.com.

If you have configured Burp's proxy listener correctly, and you haven't installed Burp's self-signed Certificate Authority (CA) certificate, yet, then the browser may throw an "invalid security certificate" error with the message "...issuer certificate is unknown". Click on the "Advanced" button to see error details.

If you received "SEC_ERROR_UNKNOWN_ISSUER" error from the browser, navigate to http://burp.

Click on "CA Certificate" link to download the "cacert.der" file.

In the Firefox browser, go to "Preferences", search for the term "certificate", and click on "View Certificates" button.

In the "Certificate Manager" window, click on "Import" button and select the downloaded "cacert.der" file.

In the "Downloading Certificate" window prompt, select checkboxes as shown in following image and click on "Ok".

Access secure websites, e.g., "https://www.example.com", without encountering the "SEC_ERROR_UNKNOWN_ISSUER" error.