Based on the details unearthed by affected users, the add-on was developed by Mozilla's Shield Studies program, a platform available on all Firefox channels that gives you a way to test features before they're released. Some Shield studies ask for your permission to opt in, others automatically make their way to your browser and require you to actively opt out. Problem is, some weren't even aware that they're part of the Shield program, so they had no idea where the extension could've come from.
Mozilla is now facing backlash for installing the add-on without people's consent, especially since it always stresses how important users' privacy is to the organization. In fact, in the page explaining what Looking Glass is, Mozilla wrote:
"The Mr. Robot series centers around the theme of online privacy and security. 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."
Despite the troubling way the extension was installed, Mozilla said it doesn't do anything to your system until you opt into the Alternate Reality Game, the immersive experience the organization designed to take fans into the Mr. Robot universe. Those who couldn't care less about the show can kill the extension by typing about:addons in their address bar and removing Looking Glass.
Update: A Mozilla rep reached out and told Engadget:
"Our goal with the custom experience we created with Mr. Robot was to engage our users in a fun and unique way. 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."