This is very nice, but the system LC_TIME locale (and whatever the equivalent is on Windows) exists for a reason. I hope Mozilla will listen to the feedback in all these comments and consider basing the picker type on the system time locale – not on the language locale, and certainly not on the locale of the downloaded binary.

I use most if not all software in English, but I prefer seeing dates in the ISO format, weeks starting from Mondays and times in a 24 hour format.