Two weeks ago, users reported a user agent string (UA string) to Mozilla about a site compatibility issue.

This user uses the latest Firefox Nightly 110, but when visiting BestBuy.com, it shows that the browser is no longer supported, and prompts to download the latest versions of Chrome, Firefox, and Microsoft Edge.

Investigation by the Firefox developers found that this issue is related to the browser’s UA. The UA information for Firefox 110 is as follows:

Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:110.0) Gecko/20100101 Firefox/110

And the UA information of IE 11 is:

Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; AS; rv:11.0) like Gecko

Since some websites treat rv:110.0 as the same as rv:11.0 when recognizing UA, Firefox 11 is recognized as IE 11 which is no longer supported.

Mozilla’s current solution to this problem is to freeze the UA, that is, use rv:109.0 until it reaches rv:120.0:

Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0) Gecko/20100101 Firefox/110

According to the release plan, Firefox 120 will be released on September 25, 2023, so the UA freeze will last for 9 months.