java crashes with firefox on 10.6.8

Hi,

I have problems with Java applets on Firefox 5.0. Applets are able to open but always crash after a few seconds. I updated my java firefox plugin to 13.8.3, but it still does the same thing.
I am using the macos x 10.6.8 version.

Does anyone there had the same issue? if yes, any good tip to fix it?
Thanks a lot!
Cheers