The Privatetab.org domain belongs to a dubious search engine that is known to be promoted by various PUPs (Potentially Unwanted Programs) online. A PUP affiliated with the propagation of the Privatetab.org search engine is known to be a dodgy Web browser add-on compatible with the Mozilla Firefox and Google Chrome browsers. The Web browser extension involved in the spreading of the Privatetab.org search engine tends to pose as a tool that helps users protect their identity online. It also would claim to be a helpful tool that would aid users in protecting their search queries from the watchful eyes of advertisement companies. However, none of this is true.
Users who install the dubious Web browser extension in question will have their default search engine replaced with the Privatetab.org search engine. Since no legitimate Web browser add-on would alter your settings behind your back, it makes sense that this extension has been classified as a PUP.
It is not recommended to use the Privatetab.org search engine. Dodgy search engines like the one in question will not provide you with organic search results. Instead, the goal of search engines like Privatetab.org is to push sponsored content as first results in an attempt to get more clicks. We would suggest that you remove the browser extension responsible for the issue and then set your default search engine to your preferred service.