>Вот у вас в браузере стоит расширение, передающее данные SimilarWeb? Или, может, вы знаете вы кого-нибудь, у кого оно стоит?

И как же вы это можете понять? Думаете все расширения, сливающие данные, открыто это афишируют?

Про similar web есть вот такое, например, расследование:

In March 2016 we noticed a website that offered information similar to analytics sites, but had no apparent relation to the sites that were analyzed – www.similarweb.com. The data includes links clicked on a site, referrer statistics, the origin of users, and others. While this is interesting, it also raises questions about the sources of data. Based on their website they collect data from millions of devices, but the software they advertise has a number of installations that was orders of magnitude lower than that. Through analysis of their official Chrome extension we
noticed a tracking library performing real-time monitoring of all visited websites. In a subsequent analysis of the top Chrome and Firefox extensions we found the library present in both official stores. We found the same library in 42 extensions in the Chrome Web Store and five extensions
in the Mozilla store, in total over 8 million installations.

Почти уверен, что в хром сторе масса бесплатных расширений, которые монетизируются, сливая данные пользователей. В тч и similarweb.