This addon adds the CACert root certificate to the Firefox trusted root store.
You can also use certutil to add root certificates to Firefox.
The addon does not work on modern Firefox, and with Letsencrypt and the slow demise of CAcert, I don't have enough motivation to fix it. I would welcome patches though to make it work again!
Public Domain/WTFPL.
Would be nice to also add the CACert Revocation List. I was not able to find out how to do this.