Google Chrome and Firefox version 100 could kill some websites — here's how

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Millennium bug-style problem with Chrome and Firefox could break websites, but plans are afoot to sidestep issues

The Google Chrome and Mozilla Firefox logos against a teal-blue background.
(Image credit: Google/Mozilla Foundation)

Chrome, Firefox and Edge are all closing in on version 100, but this impressive landmark has an unfortunate knock-on effect that will prevent websites from loading properly without intervention.

The problem will sound familiar to anyone following the news back in the late 1990s, when the Millennium bug threatened to take computer systems offline as 1999 ticked over into 2000. 

The issue then was computer systems’ inability to read four-digit year dates, and it’s similar here: the browser “user-agent” string will switch to including a three-digit version number, and early tests show high-profile websites such as Yahoo and HBO Go struggling to cope. 

On one level, it’s quite surprising that this is only being flagged now, given the switch to double-digit version numbers caused similar issues over a decade ago, but now the problem is imminent, browser makers have stepped up their plans to respond to possible troubles. Indeed, the Chrome Developers blog published instructions on how to make your website compatible with three-digit browser versions last November. 

But it would be optimistic to assume that most webmasters will put fixes in place in advance of each browser hitting version 100, so contingency plans are in place as outlined here. If Firefox encounters a bug, it will simply lie and claim the user is running version 99, while Chrome will use a flag to freeze the official version number at 99, and then put the real version information in a minor bit of the user-agent string. 

Microsoft hasn’t outlined specific plans to sidestep issues, but it has included a flag to force version 100 in the user-agent string for people to test with. 

Alan Martin
Alan Martin

Freelance contributor Alan has been writing about tech for over a decade, covering phones, drones and everything in between. Previously Deputy Editor of tech site Alphr, his words are found all over the web and in the occasional magazine too. When not weighing up the pros and cons of the latest smartwatch, you'll probably find him tackling his ever-growing games backlog. He also handles all the Wordle coverage on Tom's Guide and has been playing the addictive NYT game for the last several years in an effort to keep his streak forever intact.