Update:
I am using the JDK IcedTea plugin and it was disabled but shouldn’t have been. This and other problems have been fixed but you may have to do some cleaning to get the fix to work.
Apple users see this link.

Source: TidBITS Safe Computing – 10 Apr 2012
 Fix Firefox to Show Updated Java Plug-In

Others (this says Windows but worked on my Linux version):

  This is fixed in production now. The block should now work as a softblock.

If you had your plugin disabled here’s how to re-enable it:
1) Open about:support.
2) Look for the Profile Directory entry and click on the button next to it in order to open it.
3) Look for blocklist.xml and delete it.
4) Open about:addons
5) Enable the plugin again.

In a day or so (when the blocklist is reloaded) you’ll see a new warning about the plugin (if you’re using a vulnerable version), which you should be able to ignore. Even if you accidentally disable it again, you can follow steps 4 and 5 and this should correct the problem permanently.

There is no need to change any settings. Changing blocklist preferences can make your system very insecure and we strongly recommend against it. “

Source: 739955 – [Windows] Blocklist vulnerable jre versions pre update 31 due to security issue
Address : bugzilla.mozilla.org/show_bug.cgi?id=739955#c65