You can try this:
In the FF url bar, type in
then type in "java" in the filter box. Look for the line below. On my system, it's set to false.
If yours is set to "true", then right click on that line and select Toggle, which will change it to "false", which is what mine is set to. Or, if it's set to "false", set it to "true" to see if you get some option in Preferences that aren't there now allowing you to turn the java console on or off.