NoDoFollow 1.1 by zacharyfox is not available for firefox 6. Jayce from jayceooi.com changed the installer to make it works on Firefox 6, firefox 7, firefox 8.
1.Download new NoDoFollow 1.5 plugin by Jayce from here
2.Install it and restart firefox.
3.If you want to check a blog is do-follow or no-follow, right click on any part of your webpage in firefox and click NoDoFollow. Do-Follow links turns BLUE and No-Follow turns RED.
NoDoFollow 1.5 plugin source