It seems that the privacy community has a love/hate relationship with Firefox and Mozilla in general. Some will say it is because of their out-of-the-box privacy settings and others will say it is because of the Mozilla organization's “pro-censorship” stance.

These include constant removal of features, bad coding paradigms, poor memory management, and hidden telemetry. In short, Firefox simply doesn't work that well anymore for developers or ordinary users who just want to use their browser, thank you very much.

If you click the Advanced button on a "Warning: Potential Security Risk Ahead" error page and you see the error code SEC_ERROR_UNKNOWN_ISSUER or MOZILLA_PKIX_ERROR_MITM_DETECTED, it means that the provided certificate was issued by a certificate authority that is not known by Firefox and, therefore, cannot be trusted ...

People look for software that's easier and feels good, and Firefox simply isn't smooth, doesn't offer as many features out of the box (even some common features), and it doesn't work as well. You might be a power user, but most people don't want to have to jump through the extra hoops.