Developing software with a minimum of bugs is not easy, especially because you have to look ahead, and above all anticipate potential future problems. But the developers of Firefox and Chrome had obviously not considered that version 100 of their browser could cause compatibility problems with certain websites.
Chrome and Firefox, which will reach version 100 in a few weeks, may cause some websites to crash simply because of their three-digit version number.
99 otherwise nothing
The key to this problem is in fact entirely based on the version number of browsers and their “User agent”. This piece of code sent by browsers to websites allows them to identify them and know their version number in order to adapt their display to the functions offered by each browser. Unfortunately, to identify the version number of browsers, some sites use a system that is only interested in the first number of the version number, so far limited to two digits. And, much like the now famous Year 2000 bug, these websites know how to identify browsers up to their two-digit version of 99, but are unable to recognize version numbers beyond that. In other words, they cannot read the number 100.
Google and Mozilla are working on a solution
This problem has been known and identified for months by Google and Mozilla who are both working to try to find solutions to limit the breakage. Current versions of Chrome, Edge and Firefox already include a flag (an experimental function) that can be activated to simulate that they are in version 100 in order to allow developers of potentially affected websites to test their site and fix the problem before it officially happens.
Until a permanent solution is found, Mozilla could temporarily freeze Firefox’s version number at 99, while Google plans to modify Chrome’s user agent string by starting it with 99 and adding it behind it. the actual browser version number.
Source: Mozilla