Donations have a nice way of revealing value misalignment between the donating and receiving party. Like e.g. I'd like to donate to Firefox, maaaybe Thunderbird, but not Pocket or diversity campaigns. Money is technically fungible, but that works only if it's enough of unassigned money - if N dollars get donated "for Firefox" and then Firefox development receives less than N dollars, I'd feel cheated as a "for Firefox" donor.
Anyway, I can see this being inconvenient for Mozilla. But it is desirable for some users. I don't see how it is defeatist, and it's not unfair either - donors can say, "we want to give money for specific purposes", and Mozilla can accept or refuse. Neither outcome is unfair to anyone; donations are still voluntary transactions.