Mozilla adjusts Firefox user agent due to confusion with Internet Explorer

Mozilla is going to change Firefox’s user agent because websites sometimes mistake the browser for Internet Explorer 11. This happens due to incorrect configurations on some sites, but Mozilla thinks the problem is so big that it completely adjusts the user agent.

The browser developers have voted for a change which temporarily changes the user agent in Firefox. In the current versions of the browser, that user agent is still marked as rv:110.0 for Firefox 110 or rv:119 for Firefox 119. That causes problems on some websites, the developers write. Those websites check a visitor’s user agent and may adjust the site accordingly. Those websites do that for Internet Explorer users, for which they then load an alternative UI.

However, Internet Explorer 11 has a user agent similar to Firefox. For IE11 that is rv:11.0. Websites that check for user agents sometimes mistake Firefox’s rv:110.0 for IE11’s rv:11.0. They then present a ‘wrong’ version of the website.

For that reason, the problem only occurs with Firefox versions 110 to 119. The developers have decided to temporarily discontinue the user agent for those versions. Versions rv:110 through rv:119 now use the user agent rv:109. The original user agent will be restored when Firefox reaches version 120. It is scheduled for November this year.