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Solution is already given here.
http://softwaretestingboard.com/qa/128/performing-parallel-execution-selenium-notconnectedexception
There are three possible solutions
1.) You need to upgrade your selenium libraries.
2.) Make sure that Symantec plugin is uninstalled.
3.) Downgrade your firefox version.
here we do not have complete log. So, not sure which plugin is disturbing your execution. I would suggest you to remove all the plugins you have installed in firefox.
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