Go to Firefox > Add-ons. Select Plugins. This lists plugins Firefox has in the system. This is separate from the add-ons. Just above the first plugin is the Check to see if your plugins are up to date link. left click it.

I am trying to get an older worker of Java working, not update to the current version.

Does Oracle's installer allow you to have multiple versions? Assuming so, Firefox probably will block it, as older JRE's are considered insecure. In some cases, a blocked plugin can be enabled on an "Ask to Activate" basis. But in some cases, it simply can't be enabled at all. The control to switch between Never Activate and Ask to Activate is on the Add-ons page, plugins category, as noted by FredMcD. Disabled plugins cluster at the bottom of the list.

I was able to install the older JRE version. jre6 and jre7 will install to different directories on their own, so no trouble there. The problem seems to be how to locate the older Firefox Java plugin for 6 (and deal with the blocking of it).

Usually the installer will create a registry entry, and Firefox will find it in its routine registry scan. It then should appear on the Add-ons page in the Plugins category. Did you find it there?