New locale fingerprinting capabilities in FF31ESR
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Various element properties (https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input#attr-type) as well as the String, Date, and Number objects now expose locale-specific functions.
Also, the Intl object exposes locale information: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl
If extensions.torbutton.spoof_english is set, these functions should all behave as if the locale was en_US.