This long term tracking cookies used by Google, YouTube, ebay and many other domains are used generally for showing selected advertisements or contents which have been moulded to your interest, but it has a negative side also.
Better Privacy is an add-on to Firefox browser that serves to protect against not deletable long-term cookies. With BetterPrivacy it is possible to review, protect and delete new/old Flash-cookies individually. Users who wish to manage all cookies manually can disable the automatic functions.
BetterPrivacy also protects against 'DOM Storage' long term tracking, a browser feature which has been granted by the major browser manufactures.(via)
More information and download of BetterPrivacy