I had similar problem in Ubuntu 8.04 and the problem is not Sametime it is that Java is not correctly installed in Firefox. I followed the instructions in the following links and this solved the problem with Java not properly installed in Firefox.
http://maketecheasier.com/java-not-working-on-your-firefox-3-try-this-simple-fix/2008/07/10
http://www.shanktified.com/archives/solved-java-not-working-in-firefox-with-ubuntu-hardy-804
It these don't work try relink Suns java into Firefox
http://blog.eirikhoem.net/index.php/2008/04/30/firefox-java-problem-with-ubuntu-804-solved/
http://ubuntuforums.org/showthread.php?t=55370
Of course you have to do the linking depending on where you installed on your machine.
The others answering your questions have all the links to downloading Java.
Regards
Mats