FF version teste: FF9/FF10
Build: 8-b255-2012-02-15_168
It has been noticed sometimes on unix applets fails to laod by throwing following exception:

java.security.PrivilegedActionException: java.io.FileNotFoundException: /.java/deployment/cache/6.0/54/1319b576-4ef3c723 (No such file or directory)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.deploy.cache.DeployCacheHandler.get(Unknown Source)
    at sun.net.www.protocol.http.HttpURLConnection.plainConnect(Unknown Source)
    at sun.net.www.protocol.http.HttpURLConnection.connect(Unknown Source)
    at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown Source)
    at java.net.URL.openStream(Unknown Source)
    at sun.plugin2.applet.Applet2ClassLoader.getResourceAsStream(Unknown Source)
    at com.sun.deploy.uitoolkit.impl.awt.AWTAppletAdapter$14.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.deploy.uitoolkit.impl.awt.AWTAppletAdapter.findAppletJDKLevel(Unknown Source)
    at com.sun.deploy.uitoolkit.impl.awt.AWTAppletAdapter.setApplet(Unknown Source)
    at com.sun.deploy.uitoolkit.impl.awt.AWTAppletAdapter.instantiateApplet(Unknown Source)
    at sun.plugin2.applet.Plugin2Manager.initAppletAdapter(Unknown Source)
    at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.io.FileNotFoundException: /.java/deployment/cache/6.0/54/1319b576-4ef3c723 (No such file or directory)
    at java.io.FileInputStream.open(Native Method)
    at java.io.FileInputStream.<init>(Unknown Source)
    at com.sun.deploy.cache.DeployCacheHandler$2.run(Unknown Source)

It's hard to reproduce but issue can be seen randomly as part of nightly testing