As many extensions for firefox 3 rc1 is not yet compatible, i have been searching for updates and i found this post over on google groups:
If you want to run GM in FF3.0RC1 please install this RC1 of GM 0.8: http://arantius.info/gm/greasemonkey-0.8.20080505.0.xpi This would be the publicly available build, but Aaron (the lead developer) appears to be MIA. I’m rather sure he’ll have it up on AMO once he’s back.
I have installed the extension, and it’s working great! 
You can download it here: greasemonkey-0.8.20080505.0.xpi
Greasemonkey / Fifefox 3 RC1 - greasemonkey-users | Google Grupper
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