I don't keep close track of this, but as far as I remember they haven't tried donations that go only to Firefox/Thunderbird/etc of the person's choice, instead of Mozilla as a whole. That's what people always claim they want in these threads. I doubt donations would be enough, but I think doing it like that would at least be a step in a direction people like instead of are annoyed by, as long as they don't go nagging like Wikipedia.