He means X Windows, I suppose, the graphical environment in Linux. I assume you meant latest sdk, not jdk (jdk is only the runtime not development files) since you have javac. You should also have a program called appletviewer that installs with the sdk, try running it like this:
Code:
appletviewer appletpage.html
Instructions on installing Java in Firefox:
http://www.mozilla.org/support/firefox/faq#q2.2