Mozilla Mailnews FAQ

How do I change the display structure of the date on messages?

By default, no date is shown on messages that are dated the same day that you are reading it. This can be changed by adding the following lines to your user.js:

user_pref("mail.ui.display.dateformat.today", 2);

Or you can enter about:config in the location bar.
Add the Integer preference mail.ui.display.dateformat.today.
Set the value to 2.

Likewise, this can be set for messages sent that week:

user_pref("mail.ui.display.dateformat.thisweek", 2);

Or you can enter about:config in the location bar.
Add the Integer preference mail.ui.display.dateformat.thisweek.
Set the value to 2.

You can also set the default:

user_pref("mail.ui.display.dateformat.default", 2);

Or you can enter about:config in the location bar.
Add the Integer preference mail.ui.display.dateformat.default.
Set the value to 2.

Values are:
0 = 23:53
1 = Friday, 26. December 2003 23:53
2 = 26.12.2003 23:53
3 = 2003/12 23:53
4 = Fr 23:53