This problem comes from a recent Java update, and seems to affect only Firefox. An easy work-around is to use IE 7 instead. You can also fix the problem in Firefox by downgrading to a previous version of Java. To do this under Microsoft Windows, open the Control Panel and choose Add or Remove Programs. Look for Java(TM) 6 Update 3 and click on Remove to remove this update. After that, you should be able to launch tools in both IE7 and Firefox.

Alternatives

• Disable Javascript
  Some users report that the Java applet can view tool sessions when Javascript is disabled. For this to work, you must first stop and restart your browser, disable Javascript (don't disable Java), and start a tool. After the tool starts and is visible, you can actually re-enable Javascript. The problem is really due to the conditions of the initial start of the Java VM. Once it's running correctly, it works fine.
• Reinstall Java
  If all else fails, you can uninstall Java and reinstall the older 6 Update 2 release from scratch. To do this, visit the archive page on the Sun site, look for JDK/JRE - 6, choose 6 Update 2 from the pull-down menu, and click on the Go button to download.