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Major websites may stop working soon for Firefox and Chrome users

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Mozilla is warning users that when Firefox — and Google's Chrome — reach version 100, major websites may no longer identify them properly, and not work properly as a result.

Firefox is currently on version 97, while Chrome is on version 98. Once those are updated to version numbers with three digits, Mozilla says there are could be inconsistent problems across an unpredictable range of websites.

According to Mozilla, website servers examine what's called the User-Agent in order to determine which browser is being used. They then use that information to configure sites so that they display correctly.

"Without a single specification to follow," says Mozilla in a blog post, "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."

Mozilla points out that sites had to cope with a similar issue arose with the move from single- to double-digit version numbers, "so hitting the three-digit milestone is expected to cause fewer problems."

Nonetheless, Firefox and Chrome developers are running experiments, and also logging issues. Currently, the list of sites reporting bugs with a version 100 include T-Mobile, Yahoo, and Daimler.

"If the breakage is widespread and individual site interventions become unmanageable," continues the blog post, "Mozilla can temporarily freeze Firefox's major version at 99 and then test other options."

Similarly, the developers working on Google Chrome have a "backup plan to use a flag to freeze the major version at 99."