The obvious place to download Firefox is from the official Mozilla Firefox site.
That's not so obvious, though, in the pages displayed by the major search sites...
Yahoo! clearly wins this by not having any adverts above the main search results:
Searching Google for just 'firefox' shows a different site in the prime position and, given that Google is known for promoting Firefox itself, it's not hard to understand how someone could end up at the wrong site:
It's not so bad in the end though, as all the non-official sites just redirect to Google's Firefox site anyway (they get commission for the referrals).