> It doesn't use the standard localization setting. It has its own, but half of Firefox doesn't even seem to use that, because even though my System Preferences is English, and my Firefox language preference is English, and even my Firefox content preference is English, it's showing half the UI in Japanese still. The context menu is half English and half Japanese!

That sounds totally unexpected. Can you open a bug https://bugzilla.mozilla.org/enter_bug.cgi including the details from your about:support page?

Something sounds broken and I'd expect a fix.