6u5 Java installer fails to update HKLM->Software->JavaSoft->Java Runtime Environment->BrowserJavaVersion to point to the latest version installed (i.e, 1.6). Therefore, if there is a pre-existing version of JRE in the system, say, 1.5.0_13, the newly installed 6u5 won't get pick up by FF.

Besides updating this registry key to the latest version installed, upon uninstalled, the Java installer should also set this key back to the latest version left in the system.