The problem also exists with the newer Firefox 4.0.1 and Java 1.6.0_25 versions. Enabling administrative rights resolves the issue ... though this defeats the purpose of using a limited user. The underlying bug appears to be in the Firefox java plugin detection code. The issue is related to the detection code requiring write access to the HKLM\SOFTWARE\mozilla.org windows registry key. The limited user only has read access for this registry key. The behaviour was identified using the Sysinternals ProcessMonitor utility ... showed firefox calling an interface to create the registry key and getting an access denied response in the limited user case. A more targeted workaround for the issue is to grant the limited user write access on the HKLM\SOFTWARE\mozilla.org windows registry key. This avoids the need to grant administrative rights to the user and enables the java plugin to be detected. That is, it works on the 2 XP systems where the java plugin wasn't being detected in limited user accounts.