The influential European data privacy advocacy group None of Your Business (noyb) announced Wednesday that it has filed a complaint against Mozilla, developer of the Firefox browser, for quietly changing its privacy features so that it can track users’ website activities.

In July, Mozilla began using what it calls a “privacy preserving attribution” feature, which it implemented without consulting users and instead automatically installed as part of a software update, according to a noyb press release.

The shift lets advertisers monitor how effective their campaigns are, but does so without hoovering up individual users’ data.

Privacy preserving attribution does not use cookies but instead requires websites to ask Firefox to store information about “people’s ad interactions in order to receive the bundled data of multiple users,” the press release said.

Asserting that millions of people across Europe are impacted by the change, noyb said it has asked the Austrian data protection authority (DSB) to probe Mozilla’s actions. Multiple estimates show Firefox has about 3 percent of the global market share for web browsers.

While privacy experts have reportedly said the change is not particularly significant, it has incensed some users who flock to Firefox for the privacy features Mozilla has long touted.

Acknowledging that Mozilla’s new feature is not as “invasive” as most browsers’ cookie tracking, noyb nonetheless calls the shift particularly worrying because Mozilla “generally has a reputation for being a privacy-friendly alternative when most other browsers are based on Google’s Chromium.”

Mozilla did not immediately respond to a request for comment.

Privacy preserving attribution “interferes” with individuals’ data privacy rights under Europe’s tough General Data Protection Regulation, according to noyb.

“Mozilla has just bought into the narrative that the advertising industry has a right to track users by turning Firefox into an ad measurement tool,” Felix Mikolasch, data protection lawyer at noyb, said in a statement. 

“It is just a new, additional means of tracking users,” he added.

The advocacy group is well known in Europe for its privacy protecting work and its efforts led Meta to abandon its plans for harnessing user data to train AI algorithms in June. 

However, Meta reversed course on Sept. 13 after the U.K.’s data privacy regulator agreed to allow the social media company to move forward, saying that Meta had agreed to make it easier for users to say no to their data being used.