When accessing yahoo mail, antivirus browser components such as McAfee WebAdvisor block a domain called cow.huntforenenst.com from loading, and sites such as VirusTotal list this domain as malicious.
This domain attempts to load every time when accessing yahoo mail, and it uses a suspiciously large amount of storage for a browser cookie, it is embedded somewhere on the page itself or in an ad which tries to load.
There are a couple threads on reddit from other users experiencing this issue, as well as on places like the Norton antivirus forums.