It might be pretty late, but you CAN change it in firefox (as well as in forks). I just put this in my user.js

user_pref("intl.date_time.pattern_override.connector_short", "{1} {0}");
user_pref("intl.date_time.pattern_override.date_short", "yyyy-MM-dd");
user_pref("intl.date_time.pattern_override.time_short", "HH:mm");