Firefox users who run the stable version of the web browser and plan on updating to the April 2018 Update version of Windows 10 in the coming days may want to make sure the web browser is upgraded to version 59.0.3 to avoid compatibility issues.
Microsoft plans to publish download links and download options for the new feature update for Windows 10 today. In fact, some of the links are live already and users can download the ISO images to install the latest feature update for the operating system on target machines.
While Firefox will run on machines running the Windows 10 April 2018 Update, users may run into compatibility issues on websites that they open in the browser when running Windows 10 version 1803.
According to a bug report on Mozilla's bug tracking website Bugzilla, sites that use specific code throw the error message "invalid regular expression flag". Another site threw a "fatal error" on load. The errors are only thrown if Firefox 59.0.2 or earlier that run on Windows 10 version 1803 systems.
Firefox is not affected on earlier versions of Windows. The issue affects only Firefox, Stable, ESR and Developer Edition, and not other browsers such as Chrome or Edge according to Brian Duke who reported the issue to Mozilla.
I tried to replicate the issue in Pale Moon and the error was not thrown on a Windows 10 version 1803 machine.
David Major discovered why Firefox on Windows 10 version 1803 throws errors while the same version of Firefox on older versions does not.
When we're parsing the regexp flags here: https://searchfox.org/mozilla-central/rev/7ccb618f45a1398e31a086a009f87c8fd3a790b6/js/src/frontend/TokenStream.cpp#2002
After the 'g' we send EOF through ucrtbase!isalpha (via JS7_ISLET: https://searchfox.org/mozilla-central/rev/7ccb618f45a1398e31a086a009f87c8fd3a790b6/js/src/util/Text.h#39)
Under 17133.1, ucrtbase!isalpha(-1) returns true (at least in English locale), and it throws off our parser's logic. Under 16299, ucrtbase!isalpha(-1) returned false.
It is unclear how wide-reaching the issue is but it seems to be important enough for Mozilla to release an update for Firefox Stable and Firefox ESR shortly before the release of the next major version of Firefox (Firefox 60 and Firefox 60 ESR).
Mozilla plans to push the new version of Firefox today via the browser's update channels. When you download Firefox right now using the stub installer from Mozilla's website, you already get the new version 59.0.3 which has the issue fixed.
You can run an update check by selecting Help > Menu > About Firefox in the browser. You can check out the release notes here.
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