Firefox 151 switches gzip compression to Rust-powered zlib-rs

Firefox’s gzip path just crossed a line that matters far more than another crate release. In Firefox 151.0.0, Mozilla switched compression and decompression over to zlib-rs, a Rust implementation of zlib, after conversations with Mozilla engineers that began in summer 2024. That puts Rust in the middle of one of the browser’s most sensitive data paths, where every page load, transfer, and content fetch eventually runs into compression code.

The move is bigger than a straight library swap. Trifecta Tech Foundation describes zlib-rs as a Rust implementation of the widely used zlib library, available both as a drop-in-compatible C library and as a Rust crate. The project started with support from Prossimo and Tweede golf, and an early release was integrated into flate2 in 2024 before an ISRG audit. By January 27, 2026, zlib-rs had reached version 0.6 with a stable and complete API, and the foundation said more than 3,000 projects had adopted it by early 2026.

That maturity mattered because Firefox could not just pretend zlib-rs was byte-for-byte identical to stock zlib. The foundation said some compression settings use algorithms closer to zlib-ng, which can change exact output bytes and output length. Firefox had to update tests that checked precise output or rough compressed size, and Mozilla also prefixed zlib symbols with names like MOZ_Z_inflate to avoid clashes. This is the part hobby Rust developers should notice: production adoption is rarely just about faster code or safer code. It is about fitting into existing assumptions without breaking the machine around it.

The transition also hit real-world hardware in an ugly way. Trifecta Tech Foundation said Firefox’s new path triggered crashes tied to the Intel Raptor Lake bug, and the workaround required a small, auditable unsafe patch. Mozilla then shipped Firefox 152.0.1 on June 18, 2026, with a fix for frequent crashes affecting users with Intel Raptor Lake processors. That is exactly what mature Rust adoption looks like in practice: memory-safe code, plus the occasional careful escape hatch when the hardware underneath misbehaves.

Firefox 151.0 was first offered on May 19, 2026, and it also bundled user-facing changes like PDF merging and local profile backups on Linux. But the bigger signal is the one buried in the plumbing. A mainstream browser replaced a critical compression path with Rust, and it did so only after the project had been battle-tested, audited, integrated into flate2, and proven stable enough to carry real traffic. That is Rust’s quiet infrastructure win, and it is the sort that tends to spread.