My application uses an applet to login, the applet works fine with Mozilla 1.2,
Mozilla 1.4 I downloaded from iWork web site, but it doesn't work with
Mozilla 1.4 Beta.

I checked those three versions of Mozilla, Mozilla 1.2 is using Java plugin
1.4.2, Mozilla 1.4 from iWork web site is using 1.4.2_03 while Mozilla 1.4 Beta
is using Java Plugin 1.4.2_02.

It looks like that Java Plugin 1.4.2_02 is a problem.

Someone else has a similar problem in Bug 5005607.