I had the same problem with firefox-2. sun-java6-plugin packages is installed, but there is no symlink in /usr/lib/firefox/plugins for java plugin (/usr/lib/jvm/java-6-sun/jre/plugin/i386/ns7/libjavaplugin_oji.so), so I created it and everything is fine now, but this bug still appears in 8.04 official release, so let's hope it will be resolved quite fast.
I had the same problem with firefox-2. sun-java6-plugin packages is installed, but there is no symlink in /usr/lib/ firefox/ plugins for java plugin (/usr/lib/ jvm/java- 6-sun/jre/ plugin/ i386/ns7/ libjavaplugin_ oji.so) , so I created it and everything is fine now, but this bug still appears in 8.04 official release, so let's hope it will be resolved quite fast.