On the desktop, we're able to set the OS display language to, for example, English; and regional preferences to anything else (e.g. Brazil). With those settings, when I load the following HTML demo, the date is formatted according to my regional preferences (e.g. Brazil, dd/mm/yyyy). This is the expected behavior.
On Android, regional preferences are tightly coupled with the display language, with the exception of measurement units (Android 16+), first day of week and temperature. I have my display language set to English US, but I cannot choose Brazil as the region, so it remains United States. Because of this, when I load the HTML demo, the date is formatted as it does in the US (i.e. mm/dd/yyyy).
Getting Android to change would be very difficult, so I would appreciate if Firefox could provide an option to manually choose regional formats.