Firefox users noticed over the weekend that a mysterious extension called "Looking Glass" had been installed in their browsers without their knowledge. First noticed by multiple Reddit users, the strange new add-on featured an ominous phrase that did little to quell suspicions or fears: "My Reality Is Different Than Yours."

While many suspected the software to be some form of malicious malware, the add-on is actually an alternative reality tie-in game as part of a promotion for the popular USA Network show Mr Robot.

According to Mozilla, the "Looking Glass" plug-in was developed in collaboration with the Mr Robot team and is designed as a "shared experience to further your immersion into the Mr Robot universe".

"The Mr Robot series centres on the theme of online privacy and security," Mozilla said in support note. "One of the 10 guiding principles of Mozilla's mission is that individuals' security and privacy on the internet are fundamental and must not be treated as optional. The more people know about what information they are sharing online, the more they can protect their privacy."

The add-on was installed for Firefox users who allowed the browser to install and run Shield Studies - a functionality used by Mozilla to conduct user research and test features before they're released.

Once installed, the plugin makes minor changes and adds text to certain websites as clues for players of the alternate reality game. However, many unaware Firefox users were shocked and outraged over the plugin that was inserted without their knowledge or permission.

"I have no idea what it is or where it came from. I freaked out a bit and uninstalled it immediately," one Reddit user said.

Another added: "While I intentionally never opted out of Firefox studies, this certainly freaked me out a bit. Would have been fine with it had the extension description said it was part of a study and not 'MY REALITY IS DIFFERENT THAN YOURS.' That's not a good thing to have the only message to users as to what your extension is or does."

Others scoured through the plugin's code to uncover the source of the extension.

"Mozilla folks, what you did with this addon this was stupid and moronic. Most users are not programmers; most people don't watch Mr Robot; and most people are not going to waste a bunch of time tracking down stupid crap like this," another user said.

Many users voiced their exasperation on Reddit saying the creepy plugin freaked them out and accused Mozilla of abusing its power. Others vowed to opt out of Mozilla's studies or move to another browser altogether.

Dan Callahan, a Mozilla employee, tweeted that the "Looking Glass" plugin is a cool idea so long as users are aware and allowed to opt-in.

"For ARG players, the more a game is able to manifest in the real world, the better," Callahan tweeted. "By using an add-on, the experience could manifest outside of the game's domains, furthering immersion. As a former ARG player, this type of thing would be truly fantastic, so long as it's opt-in.

"Looking Glass is a really cool idea *for users who want it.* Pushing it to everyone, even in an inert state, was not good."

Following the fierce backlash from users and employees, Firefox said the extension will be moved to its add-on store and not automatically inserted into users' browsers.

"Our goal with the custom experience we created with Mr Robot was to engage our users in a fun and unique way," Mozilla's chief marketing officer Jascha Kaykas-Wolff told Gizmodo. "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."

"As a result we will be moving the Looking Glass add-on to our add-on store within the next 24 hours so Mr Robot fans can continue to solve the puzzle and the source can be viewed in a public repository."

The news comes as Mozilla is trying to promote its new, revamped Firefox Quantum browser while taking jabs at "big browser" Google in its ad campaigns.

Steve Klabnik, one of Mozilla's software developers, tweeted: "How can we claim to advocate for our users while violating their consent? And over something so trivial? We've finally been giving favourable press thanks to the wonderful work on Quantum. That all feels squandered now."