It's a Network Shield message. It filters traffic coming from all applications (not only browsers), and on all ports. For performance reasons, though, it tries a bit harder in case of the well-known HTTP ports.
Messages like:
Network Shield: blocked "DCOM Exploit" - attack from 81.178.115.162:135/tcp
are due to the RPC/DCOM exploit, which is a vulnerability that allows an attacker to gain access to the destination machine by sending a malformed packet to the DCOM service. It uses the RPC TCP port 135.
Which firewall do you use?
And, most important, is your operational system updated?
Generally, the firewall blocks this attack before the antivirus.