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Mozilla warns that Firefox and Chrome 100 versions may break some sites, including HBO Go and Yahoo, due to user-agent strings with three-digit version numbers

Mozilla is warning website developers that the upcoming Firefox 100 and Chrome 100 versions may break websites …

BleepingComputer Lawrence Abrams

Context & Ripple Effects

Browser changes have previously exposed dependencies beyond the browser itself: related coverage documented a Safe Browsing API failure that suppressed phishing warnings across mobile Chrome, Safari, and Firefox. The version-100 transition is a narrower compatibility problem, but it similarly puts site-side assumptions at the center of the user experience.

Mozilla’s warning makes the shift a test of whether major web services treat browser version strings as durable identifiers rather than inputs that can change format. HBO Go and Yahoo are named examples of the services exposed by that assumption.

First-order effects

  • HBO Go, Yahoo, and other affected site operators must update user-agent parsing before Firefox 100 and Chrome 100 reach their users, or risk blocking, misidentifying, or degrading those browser sessions.
  • Mozilla and Chrome’s release teams face compatibility support work even when the failure lies in website detection code rather than in browser functionality.

Second-order effects

  • Web development and testing teams will need to audit version-based browser rules across login, streaming, and other customer-facing flows, increasing the value of testing against upcoming browser releases.
  • Sites that repair only the three-digit case retain a brittle dependency; replacing hard version cutoffs with capability-based checks reduces repeat breakage as browsers continue to ship new versions.

Third-order effects

  • The episode reinforces that browser compatibility is governed jointly by browser vendors and web-service operators: widely deployed version heuristics can turn a routine release-number change into a cross-site reliability event.
  • If browser releases continue to invalidate version-specific rules, capability detection rather than user-agent version parsing becomes the more durable design norm for web services.

The trend: Browser release cycles are pushing web operators away from version-based identification and toward compatibility checks based on actual browser capabilities.

Related: Firefox · Mozilla · Yahoo · Mobile browser phishing-warning failure

Related Coverage

Discussion

  • @mozhacks @mozhacks on x
    Firefox and @GoogleChrome will reach version 100 in a couple of months. This has potential to cause breakage on sites that rely on identifying the browser version. This post covers the strategies that Chrome and Firefox are taking to mitigate the impact.https://hacks.mozilla.org…
  • @pierrefar Pierre Far on x
    Firefox and Chrome are about to get to version 100, and an extra digit in their user-agent string. This will break some websites, despite checking the UA string being an antipattern. Read and follow Mozilla's very helpful guidelines: https://hacks.mozilla.org/...
  • @chromiumdev @chromiumdev on x
    Major milestone: Chrome and Firefox will soon reach version 100! 💯 The version number is going up to three digits and both browsers are working on mitigating the potential impact of this change. Learn more about it and pitch in to help with testing ➡️ https://web.dev/...
  • @tomwarren Tom Warren on x
    Google, Mozilla, and Microsoft are getting ready for version 100 of Chrome, Firefox, and Edge to break some websites. Developers have been testing triple-digit user-agent strings, and there are plans in place to avoid any widespread breakages. Details: https://www.theverge.com/..…
  • @mikko @mikko on x
    It's the Year 2038 problem all over again. https://twitter.com/...
  • @zachleat Zach Leatherman on x
    the Y2K of user-agents https://twitter.com/...
  • @campuscodi Catalin Cimpanu on x
    The web version of the Y2K bug 😂 Websites that rely on UA string parsing may have issues with detecting Chrome and Firefox when they reach v100 in a couple of months because they haven't accounted for browsers with three-digit version numbers https://hacks.mozilla.org/... https:/…
  • @laurengoode Chaotic Goode on x
    Firefox's fate has much larger implications for the web as a whole...Aside from Apple's WebKit browser engine, Firefox's Gecko browser engine is the only alternative to Chromium in existence. https://www.wired.com/... via @mattburgess1
  • @lalalalinder Linda Duits on x
    “Chrome has won the desktop browser war,” says one former Firefox staff member ... Their hopes for a Firefox revival are not high." https://www.wired.com/...