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Have you tried:

– deactivating all plugins to see if this resolves the problem? If this works, re-activate the plugins one by one until you find the problematic plugin(s).

– switching to the default theme to rule out any theme-related problems?

– resetting the plugins folder by FTP or phpMyAdmin? Sometimes, an apparently inactive plugin can still cause problems.

– reading http://wordpress.org/support/topic/279132?

– disabling Google Gears if you have that running/installed within Firefox?