Mozilla warns Chrome, Firefox ‘100’ user agents may break sites

Firefox and Chrome header

Mozilla is warning website developers that the upcoming Firefox 100 and Chrome 100 versions may break websites when parsing user-agent strings containing three-digit version numbers.

A user-agent is a string used by a web browser that includes information about the software, such as the browser name, its version number, and the various technologies it uses.

When a person visits a website, the browser’s user-agent is sent along with the request for a web page. This allows the web page to check the visitor’s browser version and modify its response based on the features the browser supports.

For example, the current user-agent for Mozilla Firefox version 97 is:

Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:97.0) Gecko/20100101 Firefox/97.0

Note, if you have the Firefox ‘privacy.resistFingerprinting’ setting set to ‘True,’ your user-agent will be locked to ‘Firefox/78.0.’

For the current version of Google Chrome 98, the user-agent is:

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/98.0.4758.82 Safari/537.36

Mozilla warns of version 100 user-agent strings

In August 2021, Mozilla launched an experiment to see if the three-digit ‘Firefox/100’ user-agent string would cause problems with websites. Google soon followed with their own experiment for Chrome 100.

In both experiments, Mozilla and Google found a small number of websites that would not operate correctly when parsing a user-agent string that contained a three-digit version number.

Since then, Mozilla has been keeping track of web bugs caused by the version 100 change and has found problems on websites for HBO Go, BethesdaYahooSlack, and those created by the Duda website builder.

For the most part, these issues have ranged from the websites stating the browser is unsupported to user interface issues affecting portions of the site.

“Without a single specification to follow, different browsers have different formats for the User-Agent string, and site-specific User-Agent parsing. It’s possible that some parsing libraries may have hard-coded assumptions or bugs that don’t take into account three-digit major version numbers,” Mozilla explains in a new blog post about the upcoming user-agent changes.

“Many libraries improved the parsing logic when browsers moved to two-digit version numbers, so hitting the three-digit milestone is expected to cause fewer problems.”

Mozilla and Google will continue running experiments for version 100 user-agents until the browsers are released on March 29 for Chrome and May 3 for Firefox.

If there are issues with sites that Mozilla or Google cannot fix before these versions are released, both Google and Mozilla have backup plans ready to ensure the sites are not affected.

For Firefox, Mozilla has a site interventions mechanism to freeze the user-agent at Firefox/99 or inject CSS or other overrides to fix the bugs.

Similarly, Chrome plans to freeze the version shown in the user-agent string to 99 for problematic sites and report the actual version in another part of the user-agent string.

As for website developers and admins, Mozilla asks them to test if changing to the Firefox 100 and Chrome 100 user-agents break their websites.

This can be done by performing the following steps:

Configure Firefox Nightly to report the major version as 100

  1. Open Firefox Nightly’s Settings menu.
  2. Search for “Firefox 100” and then check the “Firefox 100 User-Agent String” option.

When enabled, the Firefox user-agent string will change to:

Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:100.0) Gecko/20100101 Firefox/100.0

Configure Chrome to report the major version as 100

  1. Go to chrome://flags/#force-major-version-to-100
  2. Set the option to `Enabled`.

When enabled, the Chrome user-agent string will change to:

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.4758.102 Safari/537.36

If a problem is found, Mozilla asks developers to file a report on so that there is ample time to address any issues.