insert search firefox profile folder?

How do I insert an xml search plugin to someones firefox profile folder if I dont know their unique ID in xp and vista? I'm trying to do this in .bat

C:\Users\USERSNAME\AppData\Roaming\Mozilla\Firefox\Profiles\UNIQUENUMBERSANDLETTERS.default\searchplugins

I might want to do this for adding a bookmark to their bookmark folder too.

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