If you ever hit the situation where a website causes your browser to hit 100% CPU usage, you should install Mozilla Firefox Profiler and then launch the Profiler Tab (Tools > Web Developer > Performance). You can then see v4.motads.com is sucking up so many resources. You can hit almost any major site, and see these requests.
I found the reason documented by the Electronic Frontier Foundation https://www.eff.org/deeplinks/2013/06/third-party-resources-nsa-leaks
Also, you should look at https://panopticlick.eff.org/index.php?action=log&js=yes and see how trackable you are