Mozilla products including the Firefox browser will stop trusting SSL certificates that were issued using old root CA certificates with 1024-bit RSA keys. Mozilla has already announced that it will remove the following CA certificates from the next Firefox 36, which is scheduled to be released on February 24. A certificate authority uses one or more root certificates to sign the SSL certificates issuing them to customers, these CA digital certificates are included in operating systems, most popular web browsers and other products that implements PKI infrastructure. Mozilla wants to stress certificate authorities (CAs) and their customers to stop using 1024-bits certificates because that do not offer an adequate level of security due to technological advances.”]
Source: https://securityaffairs.co/wordpress/32902/digital-id/mozilla-phasing-out-1024ca-certificates.html