Firesheep add-on identifies users on an open network who are visiting an insecure Wi-Fi hotspot. While the tool itself is not illegal, using it may be a violation of federal wiretapping laws. Experts say there are two schools of thought, with one saying there’s no reasonable expectation of privacy in a public insecure network. But the second school of thought is that when people are accessing their social network account, they have an expectation that whatever they’re doing is governed by the privacy settings in that network.”]
Source: https://www.csoonline.com/article/2126099/is-it-legal-to-use-firesheep-at-starbucks-.html