Mozilla is warning developers that Firefox versions 100 and Chrome 100 may break websites when parsing user-agent strings that contain three-digit version numbers.
A user-agent is a stringusedby a browser that includes information about its software, such as its name, version number, and the various technologies it uses.
See also: Firefox: Websites not loading for some users – What is the solution?
When a person visits a website, the browser's user-agent is sent along with the request to display the web page. This allows the website to check the visitor's browser version and modify its response based on the features the browser supports.
For example, the user-agent of Mozilla Firefox 97 is the following:
Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:97.0) Gecko/20100101 Firefox/97.0
Mozilla warns about user-agents with three-digit version numbers
In August 2021, Mozilla launched an experiment to see if the three-digit user-agent string Firefox/100 would cause problems for websites. Similarly, Google did a similar experiment for Chrome/100.
After conducting the experiments, some websites were found to be malfunctioning when parsing user-agent strings containing three-digit version numbers. Since then, Mozilla has been tracking errors on these websites caused by the version 100 change and has identified issues on sites including HBO Go, Bethesda, Yahoo, Slack, and those built with the Duda website builder.
For the most part, these issues are mainly related to websites stating that the browser is not supported while presenting interface issues that affect parts of the website. Specifically, Mozilla states that different browsers have different formats and parsing for the User-Agent string. It is possible that some parsing libraries have bugs that result in them not counting three-digit version numbers. It also emphasizes that many changes were required to the libraries when browser versions evolved into double-digit numbers.
See also: Mozilla retires VR browser Firefox Reality
Mozilla and Google's experiments will continue until the new versions 100 for Mozilla and Google are released, on May 3 and March 29, respectively. However, if these bugs cannot be fixed before the new versions are released, the companies have already formed backup plans to ensure that websites are not affected.
Specifically, Mozilla has a workaround mechanism to freeze the user-agent in Firefox/99 or introduce CSS or other workarounds to fix the bugs. Additionally, Chrome plans to freeze the version shown in the user-agent string at 99 for sites where this is a problem and report the actual version elsewhere in the user-agent string.
In order to successfully update browsers, Mozilla is asking developers to test whether changing to user-agent Firefox 100 and Chrome 100 causes problems for their websites.
This can be done in the following way:
For Firefox Nightly:
- Open the Settings menu in Firefox Nightly.
- Search for “Firefox 100” and then select the “Firefox 100 User-Agent String” option.
When enabled, Firefox's User-Agent string will change to:
Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:100.0) Gecko/20100101 Firefox/100.0
For Chrome:
- Go to chrome://flags/#force-major-version-to-100
- Set the option to "Enabled".
When enabled, Chrome's 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 any issues are identified, Mozilla asks developers to submit a report to webcompat.com so that there is enough time to address any issues.
Source: bleepingcomputer.com
