Mozilla is facing significant backlash over a new feature introduced in Firefox 128 that allows websites to perform “privacy-preserving ad measurements” by default. This option, aimed at helping advertisers understand ad effectiveness without collecting individual user data, has raised concerns among users about the privacy implications.
Mozilla describes the feature in question as part of an experimental initiative to create a new web standard. The Firefox team states this initiative is designed to “help websites understand how their ads are performing without collecting data about individuals.” Traditionally, advertising measurements involve collecting and transmitting personal data, which raises privacy concerns.
Mozilla emphasizes that the new measurement method does not track individual browsing behavior. Instead, advertisers receive aggregated data that addresses basic questions about ad performance. “Advertisers only receive aggregated data that answers basic questions about the effectiveness of their ads,” Mozilla claims. Despite these assurances, some users are unhappy that the option is enabled by default. They argue that it implies companies have a right to track online activity without explicit user consent.
The controversy centers on the Privacy-Preserving Attribution Measurement API, a feature Mozilla is developing in collaboration with Meta and other entities. This API aims to provide a privacy-first design for advertisers to measure ad-driven conversions—such as whether a user who saw an ad eventually made a purchase—without resorting to intrusive tracking methods. Mozilla’s ultimate goal is a standardized attribution solution that offers a privacy-respecting alternative to more invasive practices.
The benefits of this new API are largely indirect for users. Advertising helps support the content and services users access for free. Mozilla argues that by making advertising less invasive, it can reduce the need for more objectionable tracking practices. As Mozilla notes, the API design adds noise to the aggregated data, a method known as differential privacy, to protect individual users’ contributions from being identifiable.
Despite these privacy safeguards, many users remain skeptical. “It is strictly less privacy-preserving than not implementing this ‘feature’ that has zero benefit to the user running the browser,” argues one user, highlighting the concern that any form of ad measurement, even privacy-preserving, still involves some level of tracking.
Moreover, the decision to enable the feature by default has sparked criticism. “Enabling it by default assumes that companies have a right to track people on the internet,” states a concerned user. This sentiment is echoed by many in the community who feel betrayed by Mozilla, a company historically known for its strong stance on user privacy. The debate reflects a broader tension in the tech industry over the balance between privacy and the economic realities of online advertising.
From a technical standpoint, privacy-preserving attribution works by having Firefox store an “impression” of ads a user sees. Suppose the user later visits the destination website and completes a significant action (a “conversion”). In that case, Firefox generates a report, encrypts it, and anonymously submits it using the Distributed Aggregation Protocol (DAP) to an aggregation service. Advertisers then receive only the summary of these reports, protecting individual user data.
However, the implementation has not been without its detractors. “The worst part isn’t mentioned here. I’m fine with making any tools available to the users. But enabling by default is a very different discussion,” says a user, highlighting the lack of transparency in how the feature was rolled out. Another user expressed concern over Mozilla’s ties to the advertising industry, stating, “Particularly when one of them [Google] IS an online advertising company and another one [Mozilla] is almost exclusively funded by said advertising company. They do not deserve the benefit of the doubt.”
Mozilla maintains that the feature can be easily disabled in Firefox settings. Users can opt out by navigating to the Privacy & Security panel and unchecking the option for privacy-preserving ad measurement. Despite this, many feel that the default setting should be opt-in rather than opt-out to better respect user consent.
As we reported last year, this push on advertising-based features isn’t going to help Firefox’s market share either. The browser lost 3.92% (7.4 million) of its users in twelve months and over 30% in the past four and a half years, reflecting a continuous decline.
