Browser-based botnets are the T-1000s of the DDoS world, designed for adaptive infiltration. By the time you notice that something is wrong, your perimeter has already been breached, your servers were brought down, and there is little left to do but to hang up and move on. The attack lasted for over 150 hours, during which we recorded malicious visits from over 180,000 attacking IPs worldwide. Most commonly, the attackers were using different variants of Chrome, Opera and FireFox user-agents.
Source: https://thehackernews.com/2013/11/ddos-attack-from-browser-based-botnets.html