User agent detection (or sniffing) is the mechanism used for parsing the User-Agent string and inferring physical and applicative properties about the device and its browser. But let get the record straight. User-Agent sniffing is a future fail strategy. By design, you will detect only what is known, not what will come. The space of small devices (smartphones, feature phones, tablets, watches, arduino, etc.) is a very fast-paced evolving space. The diversity in terms of physical characteristics will only increase. Updating databases and algorithms for identifying correctly is a very high maintenance task which is doomed to fail at a point in the future. Sites get abandoned, libraries are not maintained and Web sites will break just because they were not planned for the future coming devices. All of these have costs in resources and branding.

New solutions are being developed for helping people to adjust the user experience depending on the capabilities of the products, not its name. Responsive design helps to create Web sites that are adjusting for different screen sizes. Each time you detect a product or a feature, it is important to thoroughly understand why you are trying to detect this feature. You could fall in the same traps as the ones existing with user agent detection algorithms.

We have to deal on a daily basis with abusive user agent detection blocking Firefox OS and/or Firefox on Android. It is not only Mozilla products, every product and brand has to deal at a point with the fact to be excluded because they didn’t have the right token to pass an ill-coded algorithm. User agent detection leads to situation where a new player can hardly enter the market even if it has the right set of technologies. Remember that there are huge benefits to create a system which is resilient to many situations.

Some companies will be using the User-Agent string as an identifier for bypassing a pay-wall or offering specific content for a group of users during a marketing campaign. It seems to be an easy solution at first but it creates an environment easy to by-pass in spoofing the user agent.

jlj945 said

I'm using FF 70.0.1 on Mac OS 10.15 Catalina, and Firefox is reporting the useragent to be Windows. I noticed it while downloading some apps that kept defaulting to the Windows version download link. I have a dualboot with Mac OS 10.14 Mojave, and this problem does not occur on that installation. I went into about:config to see if it was changed and it wasn't. Everything that has to do with the useragent in about:config is already configured to default. I use FF almost exclusively on every computer, this is the first I've seen this happen.

Besides the useragent being modified as this article helps fix that at https://support.mozilla.org/en-US/kb/how-reset-default-user-agent-firefox

It may be possible an app is modifying what useragent the Firefox web browser is displaying as to websites. Some proxy or connection software perhaps?

On Windows you can run Firefox in a Windows compatibility mode (never needed) and the UA can be modified at least in Windows version being shown.

Not sure about on Mac OSX though.

Senali Madawala said

snip

In case you misunderstood the question asked, the OP is using Firefox on Mac OSX yet the Firefox useragent displayed to websites such as here is as if the OP is using Firefox 68.0 on Windows and is not correct.

Did you enable this preference in about:config --

privacy.resistFingerprinting

That may override true information and show sites the "most common" information instead, including the user agent string. Whether that setting really helps is debatable.