Firefox users last week started noticing that a mysterious extension called “Looking Glass” had been added to their browsers. The extension’s description included the ominous phrase “My Reality is Different Than Yours.” There was some speculation that it was a form of malware, but users quickly identified it as part of a promotion for the USA Network hacker-conspiracy show Mr. Robot.

The problem is that the extension, which transformed some text and added game clues to certain websites, was installed without user opt-in, except to a program called Shield Studies that enabled Firefox’s creators to gather data for improving the browser. That permission is reportedly enabled by default for at least some users.

Firefox users and supporters quickly voiced concern that the use of research tools for a promotional campaign undermined Firefox’s reputation as a browser that protected users’ privacy. User HohlraumHe3 wrote on Reddit: “Browsers that care about my privacy don’t install potential spyware without notifying me. What the f–k were these people thinking? Apparently it’s time to switch browsers.”

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Mozilla, according to The Verge, responded quickly to user outrage, moving the game extension to the Firefox add-on store and making its code public.

On Saturday, a Mozilla spokesperson told The Verge that “while the web extension/add-on that was sent out to Firefox users never collected any data, and had to be explicitly enabled by users playing the game before it would affect any web content, we heard from some of our users that the experience we created caused confusion.”

The apparent misstep comes as Mozilla is trying to promote its new Firefox Quantum browser, including with ads characterizing Google’s Chrome as “big browser” and positioning Firefox as a way to defend the internet from the dominance of large corporations.