I recently updated my Java installations, which include Java 1.3, 1.4, 5.0 and 6.0, to their latest versions. Today I noticed that Java Applets stopped working in Mozilla Firefox.
After verifying the Applets still work in Internet Explorer I focused my search on Firefox. I decided to reinstall first and ask questions later. After uninstalling Firefox I renamed the leftover installation directory for further investigation. After reinstallation the Applets worked fine again.
Looking into the old plugins directory of the Firefox installation there were a few Java related DLLs.
NPJava11.dll NPJava12.dll NPJava131_20.dll NPJava32.dll npoji600.dll
This is strange, I thought that Firefox pulls the plugin directly from the JRE installation. At least recent versions of Firefox should. After searching the web a bit I found the following Java bug entry: 4637784: JDK/JRE 1.4 installation: old NPOJI600.dll blocks new plugin.
Seems like installing the old JDK 1.3 brought in the old plugin which got used but had no matching JRE installation. So, deleting
npoji600.dll may have been sufficient.