April 13, 2026
Cache me if you can
How to make Firefox builds 17% faster
Firefox devs brag 17% faster builds — commenters go from “nice” to “purge 17% of code”
TLDR: Firefox devs wired a caching tool into code generation, making rebuilds notably faster with a simple tweak. The crowd split: builders cheered the time savings, skeptics said speed won’t fix Firefox’s market slide, and power users asked if this trick can tame Rust and Windows quirks — plus memes about deleting 17% of code.
Firefox’s build team slipped a clever speed hack into the kitchen: they told a cache tool to remember a big chunk of auto‑generated code, so repeat builds reuse old results instead of doing it all again. Translation: devs save time, and everyone’s edit‑compile‑test loop gets snappier. Cue the peanut gallery’s fireworks.
The party line is simple — a tiny change, a Lua script, and suddenly warm rebuilds sprint, with one dev boasting numbers dropping toward the one‑minute mark. But the comments turned it into a full‑blown spectacle. Some cheered the easy win, with one user basically shouting, “17% for free? Ship it!” Others went existential. Shevy‑java rolled in with the doompost: why polish builds when market share is the iceberg and “dinosaur‑like extinction” is looming? Drama level: Jurassic.
Meanwhile, the toolsmiths showed up. Mmastrac asked the million‑line question: can this magic cache Rust macro stuff too — especially on Windows, where results keep changing and cache hits crater? And then the meme crowd delivered: allenrb cracked, “Just purge 17% of the code,” which somehow became the thread’s unofficial slogan.
So yes, Firefox builds get faster — but the real acceleration? Spicy takes per minute in the comments.
Key Points
- •A change (Bug 2027655) enables passing a command wrapper to py_action, allowing buildcache to intercept Firefox’s WebIDL Python code generation step.
- •The WebIDL step produces deterministic outputs from .webidl files, making it a strong candidate for caching.
- •A Lua wrapper (webidl.lua) for buildcache identifies the webidl action, gathers inputs/outputs from build metadata, and uses direct_mode for hashing.
- •Benchmarks on Linux show warm builds with buildcache plus the WebIDL plugin dropping to ~1m12s, ~15 seconds faster than without the plugin.
- •Results are from single runs on one machine; authors note they are not rigorous, and more Python codegen steps may be cached similarly.