In order to intercept HTTPS traffic from Firefox, it requires extra steps to install Proxyman CA into Firefox's Trust Store.
http://proxy.man/ssl on Firefox and download the certificate to your Download folder
2. Open Preference on Firefox (CMD+,) and open View Certificate window
3. Open Authorities Tab and select Import button
4. Select Proxyman CA, which you've downloaded and Trust all.
5. Reload your page that you need intercepting. Enjoy!