Yesterday, Firefox users noticed a strange new plug-in popping up in their browsers. A new plug-in called Looking Glass found its way into each instance of the new Firefox Quantum browser. It was disabled by default, but users were still alarmed to see a plugin they hadn’t installed. When they checked to see what Looking Glass did, they found a vague and ominous description — “MY REALITY IS JUST DIFFERENT THAN YOURS” — which did little to quiet suspicions.

“I did not remember installing this add-on, [and] I would not knowingly install it,” one user wrote in the support forum. “Any explanations welcome because I can't find any reference online.”

As it turned out, Looking Glass was part of Mr. Robot’s long-running alternative reality game, a trail of clues left by writers for fans to discover. According to Mozilla’s documentation, the plug-in was designed as a “shared experience to further your immersion into the Mr Robot universe,” developed as a collaboration between Mozilla and the Mr. Robot team at USA. The description of the app itself confirms that, listing both Mozilla developers and USA executives as authors of the plug-in.

Once enabled, the plugin seems to have made only minor changes to specific websites, likely leaving more clues for players of the Mr. Robot ARG. But seeing the plugin pop-up unannounced on their computers has left many Firefox users more than a little alarmed. As one user noted on Hacker news:

There are several scary things about this: - Unknown Mozilla developers can distribute addons to users without their permission - Mozilla developers can distribute addons to users without their knowledge - Mozilla developers themselves don't realise the consequences of doing this - Experiments are not explicitly enabled by users - Opening the addons window reverts configuration changes which disable experiments - The only way to properly disable this requires fairly arcane knowledge Firefox preferences (lockpref(), which I'd never heard of until today)

Mozilla responded to the concerns by moving the Looking Glass plug-in to Firefox’s public add-on store, and making its code available in a public repository.

“Our goal with the custom experience we created with Mr. Robot was to engage our users in a fun and unique way,” a Mozilla representative said in a statement. “Real engagement also means listening to feedback. And so 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.”

Disclosure: NBC Universal, owner of USA Network, is an investor in Vox Media, The Verge’s parent company. Additionally, we are an independent editorial partner in the Mr. Robot Digital After Show, hosted by The Verge.

Update 4:42PM ET: Updated with new Mozilla statement and public repository link.