The mysterious extension only offered a vague description - “MY REALITY IS JUST DIFFERENT THAN YOURS” – leading some users to believe it was some sort of malware that had been installed.
Looking Glass was actually a part of the television shows' long running alternative reality games that leaves a trail of clues left by writers for fans to discover. The plug-in was disabled by default but once enabled seems to have made only minor changes to specific websites, likely leaving more clues for players of the Mr. Robot ARG.
Users took to various forums to express their feelings with some noting the unannounced plugin shows how unknown Mozilla developers can distribute add-ons to users without their permission or knowledge and that Mozilla doesn't realise the consequences of these actions, according to the Verge.
The promotion stunt also highlights that experiments aren't explicitly enabled by users and that the only way to properly disable this requires fairly arcane knowledge of Firefox preferences.
Mozilla responded by moving the plug-in to Firefox's public add-on store, and making its code available in a public repository and said the extension “never collected any data, and had to be explicitly enabled by users playing the game before it would affect any web content.”