Amy Barzdukas, a general manager of Internet Explorer at Microsoft claims that Firefox’s one billion downloads claim is unlikely true.
“As with any marketing statement, I’d encourage people to be somewhat skeptical about large number claims. It’s an interesting number and I have not seen the math [but] how many internet connected users are there? 1.1 billion, 1.5 billion, something in that area.”
However Mozilla did not claim that one billion people in the world are using Firefox, they are counting downloads based on repeated downloads, updates, and people with multiple computers. This why the number does not seem too far-fetched to me.