When tabbing between buttons on the message box, the focus changes to the other button, but the default appearance (black border around the button) does not change. This is inconsistent with Windows and OS/2 at least. Tabbing should cause the default appearance to move so that pressing enter presses the new button. For instance, when the confirm first comes up, pressing tab and pressing enter should press the cancel button. It does not.
See comments inside bug 82502...
cmanske, I have to humbly disaggree with your comment in bug 82502. On Windows, tabbing or moving with the arrow keys between buttons changes the focus AND the default button. So Mozilla is behaving incorrectly here. Try this. Right click on an object on Windows, select Delete to get a message box. Use the arrow keys or tab. Notice that the focus rectangle and the default rectangle move. (you can check any 4.x dialog to see this) How else do you change the default button? Mozilla should do the same at least on Windows.
*** This bug has been marked as a duplicate of 63728 ***