a Firefox user-agent reports the Gecko productSub constant 20100101
Check id gecko-productsub-hardcode
What an ordinary browser yields
A Firefox user agent comes with navigator.productSub === "20100101", the value every Gecko build hardcodes. Chrome and Safari report "20030107" and never carry a Firefox UA.
What a detector infers
navigator.productSub is a constant compiled into the engine, and the two families disagree: every Gecko build hardcodes it to "20100101" — stock Firefox, ESR, Tor Browser, LibreWolf, Firefox for Android alike — while Chrome and Safari return "20030107". So a browser whose user agent declares itself Firefox but reports the Chromium value is contradicting its own engine claim: a Chromium base wearing a Firefox coat. This is the mirror of the productSub check that gates on a Chrome UA, and it is the stronger of the two, scored critical, because the value is a compile-time constant that resistFingerprinting does not retune — no privacy Firefox trips it. The gating is precise to avoid the one false positive available: it fires only on the full Gecko user-agent shape (an rv token, a Gecko build token, and a Firefox token together), and it never compares productSub against the UA's own Gecko/ token, because on Firefox for Android that token carries the rv version, for example Gecko/126.0, which is not the productSub date and would wrongly flag every mobile Firefox. It reads two independent constants and asks only whether they name the same engine.
How to resolve it
navigator.productSub is compiled into the engine, so it cannot be chosen independently of the engine running underneath. Presenting a Firefox user agent from a Chromium base leaves productSub reporting the Chromium constant. Run the engine the user agent claims.
Read in the wild by
Camoufox issues
The Firefox anti-detect tooling tracker records a Chromium-style productSub riding a Firefox UA — the exact self-contradiction this reads.
daijro/camoufox #105, #145 (productSub hardcode)
daijro/camoufox#105Every attribution traces to a published artifact. See the sources and their limits.
Nearby checks in Navigator identity
- navigator.language === languages[0]
languages-coherenceIn a real browser navigator.language is defined as the first element of navigator.languages — one preference list, two views onto it. - the host voice roster represents the language the browser claims
locale-vs-voice-coherenceThe voice roster is installed at the operating-system level and the language preference is set in the browser, so the two are genuinely… - eval.toString().length matches the engine (V8 = 33)
etslThe string a JS engine returns for a native function's source is an implementation detail, and its length differs per engine… - navigator.productSub === 20030107 (Chromium)
product-subnavigator.productSub is a frozen legacy constant, not a live value: Chrome, Safari and Opera return "20030107" while Firefox returns… - navigator.deviceMemory is a power-of-two spec value
device-memory-boundsnavigator.deviceMemory does not report actual RAM — the spec requires the UA to quantise it to a coarse power-of-two rung before exposing… - navigator.hardwareConcurrency is plausible
hardware-concurrencyThis is an info-severity observation, not a scored failure — it reports navigator.hardwareConcurrency and only notes whether the value is an… - the machine delivers no more parallelism than it claims cores
hardware-concurrency-vs-parallelismThe plain hardwareConcurrency row can only ask whether the number looks sane, because no second API reports a core count. - the Keyboard API's presence matches the claimed engine and form factor
keyboard-api-vs-platformThe Keyboard API — navigator.keyboard, with getLayoutMap and lock — ships in desktop Chromium and nowhere else: not in Gecko, not in WebKit…
Clearcote is a browser built for fingerprint coherence
It is a Chromium fork, maintained by the same people who wrote this reference. It ships as a compiled browser rather than as a stealth script injected into someone else's — which is a description of how it is built, and is not an argument about how it behaves on this check.
This audit takes no position on how Clearcote scores on Navigator identity checks, on this one, or anywhere else. It has no baseline corpus of other people's fingerprints to rank you against and no vendor scoreboard — nearly every check is self-referential, asking one browser the same question through two independent APIs and reporting whether both answers can be true at once. It runs identically on any browser, including ours. Run it on yours and read the result yourself.
See the other checks in Navigator identity — the family gecko-productsub-hardcode belongs to.