nsDateTimeFormatUnix calls C standard lib setlocale(), which affects all threads. Luckily, all in-tree callers want the current UI locale.
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Replying to @hsivonen
... except for all the callers that are accidentally using locale-sensitive functions when they didn't mean to

Sep 9, 2016 · 5:14 PM UTC

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Replying to @davidbaron
.@davidbaron Then there is nsCollationUnix, which calls setlocale() with a locale selectef by Web content (XPath).