According to both Google and Mozilla, Firefox and Chrome are rapidly approaching version 100, which the browser makers say could break some websites. Sometimes, cranking it to 100 can do damage.
Google Chrome is scheduled to reach version 100 on March 29, 2022, which is right around the corner. Firefox has a little more time, as it'll reach version 100 on May 3, 2022.
Mozilla and Google (via Bleeping Computer) both started testing whether version 100 would cause issues, and it does appear that some pretty major websites are having problems.
Basically, when you visit a website the browser's user-agent is sent along with the request for a web page. When the browser sends a version number with three digits, it could cause all sorts of issues.
"Without a single specification to follow, different browsers have different formats for the User-Agent string, and site-specific User-Agent parsing. It's possible that some parsing libraries may have hard-coded assumptions or bugs that don't take into account three-digit major version numbers," said Mozilla in a blog post.
Both browsers will continue to experiment ahead of the release of version 100, but it seems both Mozilla and Google have plans in place to freeze the user-agent at version 99 to prevent issues. We'll have to wait and see if this ends up being a big deal when version 100 of each browser releases, but it's definitely something worth keeping an eye on.