Will Chrome or FireFox 100 Break Your Site?

Chrome is currently at version 99 and Firefox at 98

In the past, some developers to get round buggy browsers wrote specific code that targeted specific versions.

The versions are identified by the User Agent string that the browsers send in the http headers

You can see what version you’re running in Chrome by entering in chrome://version/

At the time of writing, this is the latest version

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/99.0.4844.51 Safari/537.36

What’s the issue with 3 digit User Agent Strings?
For most sites, nothing and they will continue to work fine.

The challenge is when developers wrote code that looked up the value of the user agent and deployed different code based on it’s value.

Typically they would have looked for a two digit value, and it’s unknown what will happen when it switches to 3 digits

This a list of sites that are currently experiencing an issue

For example, if you visit this page on the Manchester Evening news as Firefox 100, you’ll get a 403 error

There are 3 ways to test your site

1. Chrome UA Switcher Extension
2. Screaming Frog User Agent
3. Setting Flags in Chrome and Firefox

Chrome UA Switcher Extension
This allows you to update the user agent string and emulate a browser.

It’s available from the Chrome Store

Once installed, select the Options menu

Then add the following into the boxes and press Add

New User-agent name:
Chrome 100

New User-Agent String
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.4844.51 Safari/537.36

Select Chrome


Indicator Flag

You can now click on the icon in the toolbar

and select the Chrome and Chrome 100 option

The flag at the top will now tell you it’s changed

You can now visit your site and see if it’s working correctly

Screaming Frog Crawler
If you own this great SEO software program, there is already s blog post on Screaming Frog site that details how to change the user agent string

Set Chrome and Firefox flags
Both Chrome and Firefox allow you to change the setting to emulate this

For Chrome:
Go to chrome://flags/#force-major-version-to-100
Set the dropdown to ‘Enabled’

For Firefox:
Download and install Firefox Nightly
Open the settings and search for ‘Firefox 100’, and then enable ‘Firefox 100 User-Agent String’