I assume you're using en-US. According to this, you should be getting en-AU since they're both `en` https://firefox-source-docs.mozilla.org/intl/locale.html#regional-preferences Can you provide a screenshot of your Windows regional settings?
Bug 1583455 Comment 1 Edit History
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I assume you're using Firefox in American English (en-US). According to this, you should be getting en-AU regional settings since they're both `en` https://firefox-source-docs.mozilla.org/intl/locale.html#regional-preferences Can you provide a screenshot of your Windows regional settings?