https://bugzilla.mozilla.org/show_bug.cgi?id=858839#c21 https://groups.google.com/forum/?hl=en&fromgroups=#!search/m...
From the latter thread, Stacy Martin at Mozilla represents "Google Analytics will not correlate or report on any customer data with any other data, they will use Mozilla data only to provide and maintain the service for Mozilla, and they will not share or use it for any other purpose."
EDIT: The contractual arrangement is relevant. Section 9.3 item 2 of the IETF do-not-track draft draws has an exception for this very situation, "data obtained by a third party exclusively on behalf of and for the use of a first party".
http://tools.ietf.org/html/draft-mayer-do-not-track-00#secti...