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After I copied my ".mozilla" folder in my home folder from my previous version of Ubuntu, to restore cache, cookies and plugins, Firefox always say there's another instance running.

No other instance is running and reboot doesn't work.

Chmod or deleting some files can help? Also safemode is not working.

If I execute Firefox as root it starts and all my data is there.

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up vote 3 down vote accepted

Since running Firefox as root works and gives you access to the data you had before, this is probably a permissions problem. It's likely that the user ID that owns some or all files in your old profile is different from your current user ID. You can fix this as follows:

sudo chown $USER:$USER ~/.mozilla

Make sure Firefox is not running when you execute that command.

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Eliah, that solved my problem and I learned something new - the chown tool- Thanks! – TheGatorade May 21 '12 at 15:43

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