From the OP, one suggestion is to collect "top-level+1 domains". This don't solve the issue of a person going to "starting_a_union_inside_company_x.com", which would be a top-level domain. Niche domains don't have a large number of users and as such the users can be trivial to deanonymized. It is also rather common that domain name servers have a private and public side. Firefox could easily become a vector of leaking from the private side, possible revealing sensitive information such as unannounced products.
From the OP we can also see that they don't intend to store IP-addresses, but it will always be possible. By using a anonymity network they can reassure the user and at the same time eliminate the risk that a malicious actor in the future will silently manage to start tracking information about which websites users go to. Additional benefits is that Mozilla also won't become a target for governments, a risk that no organization can ever be safe from if they start gather information about users.
It is not enough to strike a reasonable balance between the privacy-for-users and actionable-information-for-developers. You also need to find a balance between risk management and time spent on reducing risks. What I propose primarily is that they spend a bit more time on reducing risks, as that would benefit everyone.