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Google/Mozilla starts testing Chrome/Firerox 100 user agent string


Mar 13, 2018
In March 2022, Chromium-based browsers will move from two-digit to three-digit app versions. Such a change might throw a wrench into the Internet and break compatibility on millions of websites. To prepare its browser for a major shift in version numbers, Google starts testing Chrome 100 UA or user-agent string.

In case you don't know, the user-agent string is a set of data about your device sent by a browser to a website. A website analyzes received data and optimizes its content accordingly. For example, here is the user-agent string from Microsoft Edge:
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/93.0.4577.82 Safari/537.36 Edg/93.0.961.52.
Several weeks ago, Mozilla started testing the same thing in Firefox, which is also slowly approaching version 100. If you want to help developers make sure the Internet stays intact once Chrome 100 is out (this potentially creates a big problem as more than 60% of Internet users prefer Chrome over other browsers), you can enable an experimental flag to replace the current UA with Chrome 100.
A post on You do not have permission to view the full content of this post. Log in or register now. describes the flag:
To avoid any UA string breakage when Chrome ships v100, we should add a flag to chrome://flags which flips the current major version to 100 in the User-Agent string as well as navigator.userAgent.

Enable Chrome 100 User-Agent String in Google Chrome.​

  1. Open Google Chrome, then paste the following link in the address bar: chrome://flags/#force-major-version-to-100.
  2. Select Enabled from the drop-down next to the flag name.
  3. Click the "Restart" button to relaunch the browser and apply the changes.
Once you enabled version 100 in User-Agent, use the Internet as you usually do. To restore the default settings (if you encounter troubles with loading websites), repeat the steps above and change the flag from "Enabled" to "Default." Those using Firefox can change their UA string using our dedicated article.

The Firefox 100 user agent string​

Mozilla now wants to see how websites react to user agent strings with three digits in version numbers. The first Nightly build of Firefox 100 will come out in March 2022, which means developers have plenty of time to gather feedback, ensure websites work properly with Firefox 100 and up, and make appropriate changes if needed.

To conduct the experiment and see how websites react to version 100 in the user agent string, Mozilla added a new temporary config. It lets you browse the Internet with a custom Firefox version in UA. Mozilla says it considers freezing the user agent string on Firefox 99.0 if tests show poor compatibility and bad user experience.

Enable the The Firefox 100 user agent string manually​

If you want to participate in the development and help Mozilla shape the future of their browser, do the following:

  1. Open Firefox, then type about:config in the address bar.
  2. Acknowledge potential dangers and type general.useragent.override in the search box.
  3. Click the Switch radio button, then click the button with a plus symbol.
  4. Firefox will display a text input field, where you should paste the following line: Mozilla/5.0 (Windows NT 10.0; rv:100.0) Gecko/20100101 Firefox/100.0.
  5. Click the button with a checkmark icon to save changes, then You do not have permission to view the full content of this post. Log in or register now..
You can now browse the Internet as usual. If something goes wrong and websites start crashing, become unresponsive, or misbehave, Mozilla encourages users to You do not have permission to view the full content of this post. Log in or register now..