This indicates that a system on your network is infected by Fireball Adware.
Fireball Adware can display pop-up advertisement, hijack browser traffics, and redirect search engine queries.
Any unprotected system is vulnerable.
System Compromise: Remote attackers can gain control of vulnerable systems
The signature can be set to "Block" to block the malware's communication.
Please use Anti-Virus software to scan and clean the computer.