I wasnt too sure a limited user account could install add-ons in XP pro. You certainly have the ability as an admin to lock or change permissions on any folder and file you wish on a limited user account.

My XP pro system is mothballed, so can not experiment at the moment.

Use a mozilla.cfg file in the Firefox program folder to lock prefs or specify default values.

Place a file local-settings.js in the defaults\pref folder where you also find the file channel-prefs.js to specify using mozilla.cfg.

pref("general.config.filename", "mozilla.cfg");
pref("general.config.obscure_value", 0); // use this to disable the byte-shift

See:

• http://kb.mozillazine.org/Locking_preferences

You can use these functions in mozilla.cfg:

defaultPref();  // set new default value
pref();         // set pref, but allow changes in current session
lockPref();     // lock pref, disallow changes

lockPref("xpinstall.enabled", false);