Mozilla is testing something unusual in Firefox 149: adblock-rust, the open-source ad-blocking engine developed by Brave and written in Rust. The addition hasn’t appeared in release notes and comes without a user interface or built-in filter lists. Shivan Kaul Sahib, Brave’s VP of Privacy and Security, revealed the details on his personal blog after discovering Bugzilla bug 2013888 via a link in Waterfox’s issue tracker. Waterfox is itself adopting adblock-rust, building on Firefox’s implementation.

The bug is titled straightforwardly: “Add a prototype rich content blocking engine.” Everything is still rough around the edges: disabled by default, no filters included, no UI. But it works.

How to enable it

For anyone wanting to test it on Firefox 149 or later, open about:config and set:

privacy.trackingprotection.content.protection.enabled = true

Then manually add filter lists. For EasyList and EasyPrivacy:

privacy.trackingprotection.content.protection.test_list_urls = https://easylist.to/easylist/easylist.txt|https://easylist.to/easylist/easyprivacy.txt

The engine supports two modes: actual blocking of network requests, or simple tagging that labels content without blocking it, useful for telemetry or UI development.

What this actually means

adblock-rust is compatible with uBlock Origin filter syntax and handles both request blocking and cosmetic filtering. Mozilla clarified on Reddit that this isn’t about adopting Brave’s entire ad-blocking system wholesale. Instead, they’re testing this Rust component to refine how Firefox processes tracker lists within its existing Enhanced Tracking Protection. Support for MV2 extensions, including uBlock Origin, remains unchanged with no plans to drop it.