4. Therefore, people who use installed software are more intelligent than people who use default software (statistically)
Makes sense, but that still wouldn't explain the difference between e.g. Firefox (slightly above average intelligence), a presumably installed software, and Opera (exceptionally intelligent).
Of those that install software, a larger percentage have heard of Firefox than Opera, simply as a result of more advertising. Although Chrome has more advertising power than any of them (links to it are all over Google), it lacks the grass-roots support of Firefox.
Of these separate groups, users who actively seek out a specific browser solution are software people, who have the problem solving skills necessary to do well on an IQ test. As Opera will have the largest percentage of software people, it will have the largest percentage of high-IQ folk.Note
: I'd like to add that there are more than one form of intelligence than the kind measured by the IQ test. I don't know who said it, but I think genius is what you do, not what you are.