Reproduce steps:
1.On Mercury 12, mozilla1.4(2003/11/03),jre1.4.2_02-b03, locale english.
2.Open mozilla, visiting a website with text field. For example, http://toolsuite.central.sun.com:81/qts/5tools/
3.Input anything to password field, select all in the field.
4.Reinput something is impossible. That is, if you want to make change of what you have input, you must delete them firstly.
5.The situation is same on solaris9, English locale.
6.On winNT, the function of mozilla works well.