AFAIK ,the deal with yahoo was about putting yahoo search in the front. If it was about tracking Google search data - mozilla should have at least let people known, especially with their claim at protecting privacy. And if they lie ,they risk a very strong response, especially from developers they depend on.
Also ,if such changes we're to be made, there's a decent likelihood that someone would have noticed that data leakage and told us about it.
So since mozilla is a pretty decent company, we should currently give them the benefit of the doubt.
I think the point is more that the contract with Yahoo in the USA doesn't prevent Mozilla from making deals with smaller[1] players elsewhere (which they've been doing).
[1] Smaller than Google. The search box isn't given away for free.
Also ,if such changes we're to be made, there's a decent likelihood that someone would have noticed that data leakage and told us about it.
So since mozilla is a pretty decent company, we should currently give them the benefit of the doubt.