Firefox

At a glance

• Browser class: firefox (a Firefox/<number> token in the user agent, or mozInnerScreenX on the window); firefox_fpp when Fingerprinting Protection alters canvas or WebGL readback.
• Excluded components: none in stock Firefox; 3 under Fingerprinting Protection (canvas, screen_frame, webgl_image, ruleId firefox_fpp).
• Canvas id: stable across sessions since signalsVersion 9 (the pixels are read with getImageData(), which Firefox does not randomise).
• Unavailable by design: device_memory, heap_limit, ua_client_hints, ua_versions, keyboard_layout.
• Skipped on purpose on desktop: audio_latency.
• Lab sample: none for Firefox yet.

What works

• Graphics: canvas and webgl_image with the double render and the known-answer probes, webgl_params, and webgl_renderer and webgl_vendor. In Firefox, WhiskerJS does not request WEBGL_debug_renderer_info, because Firefox prints a deprecation warning when it is requested; gl.RENDERER and gl.VENDOR already carry the unmasked strings there.
• vendor: Firefox reports an empty navigator.vendor, which WhiskerJS reports as the plain value empty.
• Offline audio, fonts (both methods), font preferences, Math, codecs, plugins (typically a fixed PDF list), locale, storage presence, permission states and the display preference queries.
• webgpu where the Firefox version and platform offer navigator.gpu; otherwise unavailable / no_api.

What is unavailable or blocked

Component	Status	Why
device_memory	unavailable / no_api	no navigator.deviceMemory
heap_limit	unavailable / no_api	no performance.memory
ua_client_hints, ua_versions	unavailable / no_api	no navigator.userAgentData
keyboard_layout	unavailable / no_api	no navigator.keyboard
audio_latency	unavailable / skipped	on desktop, creating an AudioContext without a user gesture prints a console warning

Firefox for Android has an Android user agent, so audio_latency is read there. apple_pay and private_click report the plain value none.

What WhiskerJS excludes and why

Nothing in stock Firefox. The firefox row of the exclusion table is empty.

Fingerprinting Protection (firefox_fpp)

Firefox Fingerprinting Protection randomises canvas and WebGL readback per site and session and collapses the available screen area to the screen. It is on in private windows and, since Firefox 151, is rolling out to normal windows, starting with canvas randomisation. WhiskerJS does not read the setting. It classes a Firefox as firefox_fpp when it is not resistFingerprinting and the canvas or WebGL known-answer probe came back altered. Then:

Component	Why it is left out
canvas	canvas readback is randomised per site and session
screen_frame	the available screen area is collapsed to the screen
webgl_image	WebGL readback is randomised per site and session

All three report excluded with ruleId firefox_fpp. Stock Firefox reads the known answer back exactly, also under automation and with a screen that has no taskbar, so it stays firefox. Audio noise alone does not make the class (the audio component then turns noisy on its own). A stock Firefox with a canvas-noise extension is also classed firefox_fpp, which is right for the exclusions: its canvas is randomised too. When Firefox turns Fingerprinting Protection on in normal windows, browser_class changes and so does the id, once.

Before signalsVersion 9, stock Firefox had a second problem: Firefox 155 randomises toDataURL() per session while getImageData() stays exact, and the canvas component hashed toDataURL(), so stock Firefox gave a new id in every new browser context. Since signalsVersion 9 canvas reads the pixels and the Firefox id holds across sessions. The previous id for this bump will rarely match a stored Firefox id, because the version 8 id was not stable there.

resistFingerprinting

Only when Firefox runs with privacy.resistFingerprinting does the firefox_rfp row apply, and the test for it is deliberately strict. Each trait alone (a UTC time zone, a screen the size of the window, a pixel ratio of 1) is common in stock Firefox, so all of them must hold at once, and a browser that reports navigator.webdriver as true is never classed firefox_rfp.

That strictness is the fix of signalsVersion 5: before it, stock Firefox under automation such as Playwright (UTC, screen equal to the viewport) was wrongly classed firefox_rfp and lost nine components. A Firefox run under Playwright in a test container, at signalsVersion 5, was classed firefox and raised only automation_webdriver; since signalsVersion 8 Playwright with an exposed function also raises exposed_binding. See What’s new and Firefox resistFingerprinting and Tor Browser.

Integrity flags to expect

• engine_mismatch checks that a Firefox class behaves like Gecko: a 37-character eval source and navigator.productSub 20100101. A user agent claiming Firefox in another engine raises it; so can user-agent switcher extensions.
• automation_webdriver under Selenium, Playwright or similar.
• native_tampering, prototype_lie and navigator_own_property with privacy or anti-fingerprinting add-ons.
• software_renderer where WebGL runs in software, for example llvmpipe on some Linux setups.
• exposed_binding under Playwright with an exposed function. Since signalsVersion 9 it reads window properties through their descriptors, so Firefox no longer prints deprecation warnings (Window.fullScreen, InstallTrigger).
• ua_ch_mismatch is never checked: Firefox has no navigator.userAgentData. media_query_mismatch does not run for the firefox_fpp class.
• Fingerprinting Protection’s noise is never a flag: it shows in noise_profile, canvas_readback and the excluded components.

Early lab observations

There is no lab data for Firefox yet. The early, small sample at signalsVersion 1 covered Mac Chrome, Mac Safari, iPhone Chrome and iPhone Safari only. The table below comes from the code.

Testing it yourself

Open the live demo in Firefox and check browser_class: it should say firefox, or firefox_fpp in a private window or with Fingerprinting Protection on (then noise_profile shows which probe saw noise). Run it again after a Firefox update and see which structuredId group moved. The lab mode copies one line per run. Nothing is sent anywhere.

Support table for Firefox

Expected status of every component in this browser at signalsVersion 9, from the support matrix (updated 2026-10-26). Basis "code" means expected from the library code and documentation, not yet measured; "lab" means seen in the early lab sample.

Component	Status	Basis	Note
apple_pay	none	code	No ApplePaySession: reports the plain value none.
audio	ok	code	noisy / seeded_noise if the audio known-answer probe sees noise.
audio_latency	varies	code	Skipped on desktop; Firefox for Android has an Android user agent, so it is read there.
browser_class	ok	code	Class firefox; firefox_fpp when Fingerprinting Protection alters canvas or WebGL readback.
canvas	varies	code	Excluded (firefox_fpp) when Fingerprinting Protection is on: in private windows, and in normal windows as Firefox 151 and later roll it out. Otherwise ok.
canvas_readback	varies	code	exact, or altered with Fingerprinting Protection or a canvas-noise extension.
color_depth	ok	code
color_gamut	ok	code
content_blockers	ok	code	none, or the bait names a content blocker hid.
cpu_architecture	ok	code
device_memory	unavailable	code	navigator.deviceMemory is a Chromium API. Reports unavailable / no_api.
dynamic_range	ok	code
font_preferences	ok	code
fonts	ok	code
fonts_canvas	ok	code
forced_colors	ok	code
hardware_concurrency	ok	code	Read 16 times; noisy / read_varies if the reads disagree. Some browsers cap or round the value.
heap_limit	unavailable	code	performance.memory is a Chromium API. Reports unavailable / no_api.
intl_locale	ok	code
keyboard_layout	unavailable	code	No navigator.keyboard here: unavailable / no_api.
languages	ok	code
math	ok	code
media_codecs	ok	code
noise_profile	ok	code	exact on stock Firefox; seeded for canvas or WebGL with Fingerprinting Protection or a noise extension.
pdf_viewer	ok	code
permissions	ok	code
pixel_ratio	ok	code
platform	ok	code
plugins	ok	code
prefers_contrast	ok	code
private_click	none	code	No attributionSourceId on anchors: reports the plain value none.
reduced_motion	ok	code
screen_frame	varies	code	Excluded (firefox_fpp) when Fingerprinting Protection is on: in private windows, and in normal windows as Firefox 151 and later roll it out. Otherwise ok.
screen_resolution	ok	code
speech_voices	ok	code	Digest of the voice list, or none; some browsers fill the list late.
storage	ok	code
time_zone	ok	code
touch_support	ok	code
ua_client_hints	unavailable	code	navigator.userAgentData (Client Hints) is a Chromium API. Reports unavailable / no_api.
ua_versions	unavailable	code	navigator.userAgentData (Client Hints) is a Chromium API. Reports unavailable / no_api.
vendor	ok	code
vendor_flavors	ok	code
webgl_image	varies	code	Excluded (firefox_fpp) when Fingerprinting Protection is on: in private windows, and in normal windows as Firefox 151 and later roll it out. Otherwise ok.
webgl_params	ok	code	Where WebGL 1 is available; unavailable / no_context without it.
webgl_renderer	ok	code	Read from gl.RENDERER; WEBGL_debug_renderer_info is not requested in Firefox.
webgl_vendor	ok	code	Read from gl.VENDOR; WEBGL_debug_renderer_info is not requested in Firefox.
webgpu	varies	code	Depends on browser version, operating system and GPU; needs a secure context. unavailable / no_api without navigator.gpu, no_context without an adapter.

Questions and answers

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