Mozilla and Google have disagreed about how much room browsers should leave for ad-blocking extensions, and there is a growing split over this, leaving Firefox and Chrome on opposite sides of the battle.
The split focuses on a feature called Web Request, which is used in ad blockers and is essential for any system looking to block domain sales.
Google has long had security concerns about the web request and has worked to exclude it from the latest extension standard, called Manifest V3.
But Mozilla clarified in a recent blog post that Firefox continues to support web request, which keeps the door open for the most sophisticated form of ad blocking.
Google's strategy has been heavily criticized by privacy advocates, and the Electronic Frontier Foundation has been an outright opponent. But the search company was not affected.
Firefox has a much smaller market share than Chrome, but this may be an opportunity for Mozilla's product to define itself. Google's commitment to MV3 makes a huge impact on the overall look of ad blocking across the modern web.
The changes in MV3 are part of a planned overhaul of the Chrome browser extension's specification that defines the permissions, capabilities, and system resources an extension can use.
Under current specifications - Manifest V2 - the extension can use an API feature called Web Request to monitor traffic between the browser and a website and to modify or block requests to certain domains.
The Web Request feature is important, and can be used for both beneficial and harmful purposes. Ad blocking extensions use this feature to block incoming and outgoing traffic between certain domains and the user's browser.
It also uses it to block domains that run ads and stops sending information from your browser to any one of the thousands of tracking domains that collect data about Internet users.
But the same feature can be maliciously used to hijack users' data or insert additional ads into web pages, which is one reason Google has changed the way it works in Manifest V3.
Mozilla defends privacy
The blocking version of the Web Request API has been removed under the new specification, and replaced with an API called Declarative Net Request.
Rather than monitoring all the data in a network request, the new API forces extension makers to pre-define rules about how to handle certain types of traffic, with the extension able to perform a narrower set of actions when the rule is triggered.
For some add-ons, this is a problem. Adblock Plus, one of the most popular ad blockers, has approved the MV3 changes. Although it is worth noting that the extension has a financial relationship with Google.
Google introduced the changes as a privacy, security, and performance feature. But critics see it as an attempt to limit the impact of the ad ban on a company that is funded almost entirely by advertising.
But makers of some ad-blocking and privacy-protection add-ons said the change undermines the effectiveness of their products. "Google is focused on the message of privacy by design," said Jean-Paul Schmitz, CEO of the privacy-focused browser extension Ghostery. But it still asserts its monopoly on the entire ecosystem. This comes by stifling digital privacy companies that work to give users control over their data.
The Ghostery extension is a prime example of a product that could be severely affected by Google's changes. Besides blocking advertising content, the extension analyzes communications between a website and a user's browser to look for data that can inadvertently identify a unique visitor to the site and replace it with general data before network traffic leaves the browser.
This requires the ability to modify web traffic on the go, the developers say. As a result of MV3's limitations, it is severely restricted.
The developers of ad blockers are also concerned that the effects of these changes reach far beyond Chrome. The MV3 specification is part of the Chromium project, an open source web browser created by Google that forms the basis for Chrome, Edge, Opera, Brave, and more.
Firefox allows add-ons to use more complex blocking techniques
Since Chromium supports these projects, browsers that rely on it may have to switch to the MV3 Add-ons format. These browsers' extensions will no longer be able to block ads using the web request.
As the primary developer of Chromium, Google exercises a tremendous amount of control over what browser extensions can and cannot do.
This differentiates browsers that do not rely on Chromium - notably Firefox and Safari - because they have a chance to take a different approach to extension design, and they are now in a position to set themselves apart by a more permissive approach to ad blocking.
For compatibility reasons, Mozilla continues to use most of the Manifest V3 specification in Firefox so that add-ons can be ported from Chrome with minimal changes. But Firefox continues to support web-request blocking after Google phases out it, enabling more sophisticated ad blockers to work as usual.
Mozilla has been clear in recognizing that privacy is a core value for the people who use its products. "We know that blocking content is important to Firefox users and we want to ensure that they have access to the best privacy tools available," she said. In Firefox we block tracking by default but we still allow ads to load in the browser. If users want to take the step of blocking ads entirely, we believe it is important to enable them to do so.
Google is on its way
Regardless, it looks like Google is on its way. The company said it supports the ban and aims to limit the type of data some extensions can collect. This is despite a wave of criticism from the developers of ad blockers.
"We're excited to see Mozilla support Manifest V3, which aims to make add-ons more secure for everyone," she added. Chrome supports and continues to support ad blockers. We're changing the way network request blocking works because we're making fundamental changes to how extensions work in order to improve the security and privacy features of our extension platform.
According to the search giant, it has heard positive feedback about the changes from several content blocking extension developers. It noted that it has won praise from the makers of Adblock Plus.
Firefox's stance on ad blocking is likely to encourage more users to switch to the browser. Firefox currently accounts for less than 8% of the computer browser market, compared to 67% for Chrome.
The changes will become more visible to users of any Chromium-based browser once Manifest V2 support ends in June 2023.