Where does Firefox 17.0 look for PlugIns? I installed java script jre-7u9 but it is not found when I aunch Firefox.

I am running Linux Mint 12, based on Ubuntu, and I have the Linux version of Firefox 17.0. I installed the 32 bit version of java-7-openjdk (jre1.7.0_09) in /etc. in the java-openjdk sub-folder. Firefox when I start it tells me that I should install an Add-on, suggests the this Add-on, then tells me it is installed. But when I ask which version of Java I have I am told that none is found! When I restart Firefox and the loop starts over.