1.Open a new window in Firefox and type "about:config" without the quotes and hit enter
2.Confirm that you want to continue
In the search field, type in security.enterprise_roots.enabled and hit enter
3. - you'll be left with one field
If it's marked as "false" then double-clicking it should turn it to "true"
Close the window, then re-open your website.
This error we faced due to ssl certificate installed by our organization.
Also this solution is applicable for error:
MOZILLA_PKIX_ERROR_MITM_DETECTED
