Gecko user agent string reference

This document describes the user agent string used in Firefox 4 and later and applications based on Gecko 2.0 and later. For a breakdown of changes to the string in Gecko 2.0, see Final User Agent string for Firefox 4 (blog post). See also our document on user agent sniffing and this Hacks blog post.

General form

The UA string of Firefox itself is broken down into four components:

Mozilla/5.0 (platform; rv:geckoversion) Gecko/geckotrail Firefox/firefoxversion

• Mozilla/5.0 is the general token that says the browser is Mozilla compatible, and is common to almost every browser today.
• platform describes the native platform the browser is running on (e.g. Windows, Mac, Linux or Android), and whether or not it's a mobile phone. Firefox OS phones simply say "Mobile"; the web is the platform. Note that platform can consist of multiple "; "-separated tokens. See below for further details and examples.
• rv:geckoversion indicates the release version of Gecko (such as "17.0"). In recent browsers, geckoversion is the same as firefoxversion.
• Gecko/geckotrail indicates that the browser is based on Gecko.
• On Desktop, geckotrail is the fixed string "20100101"
• Firefox/firefoxversion indicates the browser is Firefox, and provides the version (such as "17.0").
• from Firefox 10 on mobile, geckotrail is the same as firefoxversion

For other products based on Gecko, the string can take one of two forms, where the tokens have the same meaning except those noted below:

Mozilla/5.0 (platform; rv:geckoversion) Gecko/geckotrail appname/appversion
Mozilla/5.0 (platform; rv:geckoversion) Gecko/geckotrail Firefox/firefoxversion appname/appversion

• appname/appversion indicates the application name and version. For instance, this could be "Camino/2.1.1", or "SeaMonkey/2.7.1".

• Firefox/firefoxversion is an optional compatibility token that some Gecko-based browsers may choose to incorporate, to achieve maximum compatibility with websites that expect Firefox.  firefoxversion will generally represent the equivalent Firefox release corresponding to the given Gecko version. Some Gecko-based browsers may not opt into using this token; for this reason, sniffers should be looking for Gecko — not Firefox! Whether this token appears is controlled by the "general.useragent.compatMode.firefox" boolean pref.

The platform part of the UA string indicates if Firefox is running on a phone-sized or tablet device. When Firefox runs on a device that has the phone form factor, there is a Mobile; token in the platform part of the UA string. When Firefox runs on a tablet device, there is a Tablet; token in the platform part of the UA string instead. For example:

Mozilla/5.0 (Android 4.4; Mobile; rv:41.0) Gecko/41.0 Firefox/41.0
Mozilla/5.0 (Android 4.4; Tablet; rv:41.0) Gecko/41.0 Firefox/41.0

The preferred way to target content to a device form factor is to use CSS Media Queries. However, if you use UA sniffing to target content to a device form factor, please look for Mobi (to include Opera Mobile, which uses "Mobi") for the phone form factor and do not assume any correlation between "Android" and the device form factor. This way, your code will work if/when Firefox ships on other phone/tablet operating systems or Android is used for laptops. Also, please use touch detection to find touch devices rather than looking for "Mobi" or "Tablet" - there may be touch devices which are not tablets.

Windows

Windows user agents have the following variations, where x.y is the Windows NT version (for instance, Windows NT 6.1).

Macintosh

Here, x.y is the version of Mac OS X (for instance, Mac OS X 10.6).

Linux

Linux is a more diverse platform. A few common examples are given below.

Android (version 40 and below)

Android (version 41 and above)

Beginning in version 41, Firefox for Android will contain the Android version as part of the platform token. For increased interoperability, if the browser is running on a version below 4 it will report 4.4. Android versions 4 and above will report the version accurately. Note that the same Gecko—with the same capabilities—is shipped to all version of Android.

Firefox OS

Firefox for iOS

Firefox for iOS uses the default Mobile Safari UA string, with an additional FxiOS/<version> token, similar to how Chrome for iOS identifies itself.

Firefox Web Runtime

Same as desktop Firefox UA strings.

Device-specific user agent strings

Although it is strongly discouraged by Mozilla, some handset manufacturers unfortunately include a token in their device's UA string that represents their device id. If this is the case, the Firefox OS UA string will look like the device-specific string in the table above, where nnnn; is the manufacturer's code for the device (see Guidelines). Some of them we have noticed are of the form "NexusOne;", "ZTEOpen;", or "Open C;" (note that putting space is also discouraged). We provide this information to assist with your UA detection logic, but Mozilla discourages the detection of a device id in UA strings.

Here is a JavaScript regular expression that will detect all mobile devices, including devices with a device id in their UA string:

/mobi/i

The i makes it case-insensitive, and mobi matches all mobile browsers.

Firefox OS version number

While the version number for Firefox OS is not included in the UA string, it is possible to infer version information from the Gecko version number present in the UA string. Beginning with Firefox OS 1.2, each minor version should correspond to two major Gecko revisions.

Firefox OS has a 4 digit version number: X.X.X.Y. The first 2 digits are owned by the Mozilla product team and will denote versions with new features (eg: v1.1, 1.2, etc). The third digit will be incremented with regular version tags (~every 6 weeks) for security updates, and the fourth will be OEM owned.

Other Gecko-based browsers

These are some sample UA strings from other Gecko-based browsers on various platforms. Note that many of these have not yet been released on Gecko 2.0!

Implementation notes for applications, vendors, and extensions

Prior to Firefox 4 and Gecko 2.0, it was possible for extensions to add user agent parts through the general.useragent.extra.identifier preferences, (see the obsolete User Agent Strings Reference). But that has not been possible since bug 581008.

In the past, specific plug-ins, add-ons or extensions added user agent parts to notify sites they were installed. The recommended way to do this, if it's absolutely necessary (remember that it slows down every request) is to set a custom HTTP header.

See Also

• Firefox OS User Agent String (blog post w/bug 777710 reference)
• Final User Agent string for Firefox 4 (blog post)
• Recommendations on sniffing the UA string for cross-browser support
• window.navigator.userAgent
• Add Android version to Fennec UA String (bug 1169772)

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