I am not using the linux version but there is a note on java at firefox:
http://support.mozilla.com/en-US/kb/Using+the+Java+plugin+with+Firefox?bl=n&s=linux&as=q

Other software that may block Java

Even if Java is installed and enabled in Firefox, it may be blocked by other software or Firefox add-ons, such as the following:
Windows:

* The ZoneAlarm Pro software firewall can block Java. For more information on enabling Java through this program, see the ZoneAlarm Pro forum.

* The NoScript extension for Firefox will block Java. For more information, see the NoScript FAQ

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