Firefox Borrows Brave’s Adblock Core: A Stealth Move Toward Native Blocking

Mozilla slipped Brave’s adblock-rust engine into Firefox 149 last month. No fanfare. No release notes mention. Just code, vendored via the browser’s Rust tooling and wired into the network stack. It’s FOSS spotted it first, thanks to a post from Shivan Kaul Sahib, Brave’s VP of Privacy and Security.

The engine powers Brave Shields. Rust-built. MPL-2.0 licensed. It chews through network requests, applies cosmetic filters, handles scriptlet injections—even supports uBlock Origin’s syntax extensions. In Firefox, Mozilla engineer Benjamin VanderSloot landed it through Bugzilla 2013888, titled “Add a prototype rich content blocking engine.” He vendored the crate, audited dependencies like flatbuffers and seahash, then integrated it via a ContentClassifierService with C++ wrappers hooked to AsyncUrlChannelClassifier.

Disabled by default. No UI toggle. No bundled lists. Users hunt prefs in about:config. Flip privacy.trackingprotection.content.protection.enabled to true. Pipe in EasyList and EasyPrivacy: privacy.trackingprotection.content.protection.test_list_urls = https://easylist.to/easylist/easylist.txt|https://easylist.to/easylist/easyprivacy.txt. Reload. Ads vanish on test sites like Yahoo—banners shrink to “Advertisement” stubs, layout intact. VanderSloot noted in the bug log that it sits alongside Firefox’s existing URL classifier, pref-controlled for experiments. Linuxiac called it foundational work, the first native filtering engine in Firefox’s codebase.

Why now? Privacy arms race heats up.

Brave’s engine isn’t new. Overhauled in January with FlatBuffers, slashing memory 75%—45MB savings across platforms. Brave’s blog touts it as native advantage over extensions, immune to Manifest V3 limits crippling uBlock Origin on Chromium. Firefox dodged that bullet, keeping full uBlock support. But extensions run sandboxed, spawn processes, lag on updates. Native? Faster. Tighter. Runs in the main thread.

Kaul Sahib wrote on his blog: “Pretty exciting to see them finally start taking ad & tracker blocking seriously (Firefox still doesn’t remove tracking query parameters).” Experiment only, he added—no UI, no lists, no press. Yet browsers pile on. Waterfox builds atop it for 2026, ditching extension woes. “It’s faster, more tightly integrated, and doesn’t depend on a separate extension process,” their blog explains. MPL-2.0 matches; uBlock’s GPLv3 wouldn’t. They whitelist search text ads for revenue, toggleable.

Mozilla’s Enhanced Tracking Protection blocks some trackers. But ads? Users grab uBlock Origin—90 million downloads. YouTube cracks down anyway. Native blocking sidesteps that. No extension needed. Scales better on low-end hardware. Winaero sees a shift in content handling, still experimental behind flags.

And the open-source ripple. adblock-rust on GitHub: 1,100+ commits. Servo crates for CSS selectors—shared with Firefox. Perplexity’s Comet uses it too, per Kaul. Forks like Waterfox accelerate. Reddit buzzes: r/browsers calls it exciting; some wonder if it’ll match uBlock fully.

Mozilla stays mum. No official word. Bug fixed two months back, pushed after backouts for leaks and tests. Prefs hint at modes: protection blocks; annotation tags for telemetry. Devs tweak. privacy.trackingprotection.content.testing fires notifications.

Short term? Tinkerers enable it. Test on fresh profiles—about:config tweaks cache lists on change. Long term? UI lands. Lists bundle. Strict mode rivals Brave. Firefox claws market share—users flee Chrome’s ad-push. Privacy wins without extensions.

But gaps remain. No tracking param stripping, Kaul notes. Cosmetic filters? Prototype supports, but unpolished. uBlock users won’t ditch yet; native must prove parity.

Quiet integration. Bold signal. Browsers converge on built-in shields. Brave’s gift keeps giving.