Firefox user agent pattern
Firefox's user agent is built around the Gecko engine token and a trailing Firefox product token. Compared with other browsers it has been relatively stable and predictable, which makes it straightforward to recognise. This page covers the pattern and how to tell Firefox apart from WebKit and Chromium clients.
The Firefox UA pattern
Firefox's user agent contains a Gecko token (often as a fixed Gecko/20100101-style compatibility marker) and a trailing Firefox product token with the release version. The platform descriptor in parentheses indicates the operating system.
Unlike Chrome and Safari, Firefox does not carry an AppleWebKit token, which is one of the clearest ways to separate Gecko-based Firefox from the WebKit/Blink families.
- Contains a Gecko token and a Firefox product token
- No AppleWebKit token (unlike Chrome and Safari)
- Platform descriptor indicates the OS
Relatively stable, still client-controlled
Firefox's user agent has historically been more stable and predictable than some peers, which makes pattern matching easier. Even so, it remains a client-controlled string: a scraper can copy it, and Mozilla can change it.
Match on the stable Gecko and Firefox tokens rather than a full version string, and never treat the string as proof of identity.
How it appears in analytics and logs
A user agent containing the Gecko and Firefox tokens is typically Firefox. The absence of an AppleWebKit token is a strong cue separating it from Chrome and Safari families.
Diagnostic use case
Recognise Firefox traffic in logs and analytics by its Gecko and Firefox tokens, and distinguish it from Chromium and WebKit clients that share the AppleWebKit lineage.
What WebmasterID can help detect
WebmasterID recognises Firefox and Gecko-based clients server-side from their pattern and records a coarse browser category, robust to version changes.
Common mistakes
- Assuming any string with a Gecko-like marker is Firefox without checking for the Firefox token.
- Hard-coding a full Firefox version string that changes each release.
- Treating the Firefox UA as proof the client is a real browser.
Privacy and accuracy notes
Firefox's UA describes the browser, not a person. Mozilla has worked to limit UA entropy for privacy. WebmasterID stores a coarse browser category rather than the raw visitor string.
Related pages
- Browser user agents: how to read them
A browser user-agent string packs several tokens into one line: a legacy Mozilla prefix, a rendering-engine signature, the platform, and the browser itself. This page explains each part so you can read a UA without over-reading it, because the contents are client-controlled and can be copied by any client.
- Chrome user agent and its quirks
Chrome's user-agent string is full of historical artefacts: it claims AppleWebKit and Safari for compatibility even though Chrome uses the Blink engine. Google has also reduced the detail Chrome exposes in the UA, moving fine-grained information into User-Agent Client Hints. This page explains the pattern and the quirks.
- Bot intelligence
Categorise browsers, crawlers, and automation deterministically.
Sources and verification notes
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.