[luckylion]

Which requires the user to trust Google to a) honor that agreement (somewhat simple, though we don't know the actual terms, i.e. what's on the line for Google) and b) not have bugs in their systems that accidentally leak information (to their own profiling services or third parties), and if they trust them on this, why not trust them in general when it comes to "we won't use your information for anything nefarious". Anti-Ad/Tracking-Plugins being among the most popular suggests that a lot of Mozilla's users don't want to rely on trust.

My bank argues the same way and uses Google Analytics to track their visitors, including inside the online banking system. Fine, so they trust Google to honor agreements and not connect profiles, but I'd still prefer Google to simply not know when and how often I'm logging in to check my account balance.

It's good that Mozilla goes the extra mile to get a custom contract, but I believe that most people aren't expecting a self-proclaimed privacy champion to use an anti-privacy-service by one of the largest corporate enemies of privacy. Explicit opt-in would be the right thing to do here.