From MozillaZine Knowledge Base

How do I change the date format?

Mozilla applications use the operating system date format so that date formats are consistent across the screen.

To change your operating system date format, see: Configuring the date/time system settings on your computer.

See also Sunbird/Lightning 0.3 issue: Week view and Day view heading date format does not match system.

See also tip: Narrow date columns.