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 say "Mobile"; the web is the platform. Note that platform can consist of multiple "; "-separated tokens. See below for further details and examples.

  Note: Though fixed in Firefox 69, previous 32-bit versions of Firefox running on 64-bit processors would report that the system is using a 32-bit CPU.

• 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!

Prior to Firefox 4 and Gecko 2.0, it was possible for extensions and plug-ins to add user agent parts, but that has not been possible since bug 581008.

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", since there may be touch devices which are not tablets.

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

Here, x.y is the version of Mac OS X (for instance, Mac OS X 10.15). Starting in Firefox 87, Firefox caps the reported Mac OS X version number to 10.15, so macOS 11.0 Big Sur and later will be reported as "10.15" in the User-Agent string.

Note that Firefox no longer officially supports Mac OS X on PowerPC.

Linux is a more diverse platform. Your distribution of Linux might include an extension that changes your user-agent. A few common examples are given below.

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 versions of Android.

From version 1, Focus is powered by Android WebView and uses the following user agent string format:

Mozilla/5.0 (Linux; <Android Version> <Build Tag etc.>) AppleWebKit/<WebKit Rev> (KHTML, like Gecko) Version/4.0 Focus/<focusversion> Chrome/<Chrome Rev> Mobile Safari/<WebKit Rev>

Tablet versions on WebView mirror mobile, but do not contain a Mobile token.

Starting in Version 6, users can opt into using a GeckoView-based Focus for Android with a hidden preference: it uses a GeckoView UA string to advertise Gecko compatibility.

Since version 4.1, Klar for Android uses the same UA string as Focus for Android. Before version 4.1, it sent a Klar/<version> product/version token.

Version 7 of Focus for iOS uses a user agent string with the following format:

Mozilla/5.0 (iPhone; CPU iPhone OS 12_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) FxiOS/7.0.4 Mobile/16B91 Safari/605.1.15

Note: this user agent was retrieved from an iPhone XR simulator and may be different on device.

Version 3 (and probably earlier) of Firefox for Fire TV use a user agent string with the following format:

Mozilla/5.0 (Linux; <Android version>) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Focus/<firefoxversion> Chrome/<Chrome Rev> Safari/<WebKit Rev>

From version 1.1, Firefox for Echo Show uses a user agent string with the following format:

Mozilla/5.0 (Linux; <Android version>) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Focus/<firefoxversion> Chrome/<Chrome Rev> Safari/<WebKit Rev>

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.

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.

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

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

Regarding the deviation on iPad form factor, see this issue.

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!

• 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)