firefox3 addon doesn't get installed!!!

Hi,
i am using Firefox3.when i about to install addons like NoScript,AdBlockerplus,Greasemonkey.It shows following error.
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Firefox could not install the file at

*addons.mozilla.org/en-US/firefox/downloads/file/30326/greasemonkey-0.8.20080609.0-fx.xpi

because: Unexpected installation error
Review the Error Console log for more details.
-203
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,So no addon r installed!!.Pl Help me in fixing addon!