You actually have to reinstall firefox using a different download to get `new Date(dateString)` to use a different date format.
This method should not be used, since it relies on the
new Dateconstructor, which behaves very differently across browsers and locales. Use
dateISOinstead or one of the locale specific methods (in localizations/ and additional-methods.js).
So if you're using chocolatey or boxstarter like I am (my boxstarter script) you need to add the locale flag: https://chocolatey.org/packages/firefox
choco install Firefox -packageParameters "l=en-GB".
Reinstalling the package with --force is sufficient to change it.