IE surfers were found to have an average IQ lower than people using Chrome, Firefox and Safari. Users of Camino and Opera rated highest.
The report has sparked anger from IE supporters, who have threatened AptiQuant with legal action.
Researchers gave over 100,000 web surfers a free online IQ test. Scores were stored in a database along with each person's web browser data.
The results suggested that Internet Explorer surfers had an average IQ in the low eighties. Chrome, Firefox and Safari rated over 100, while minority browsers Opera and Camino had an "exceptionally higher" score of over 120.
AptiQuant stressed that using IE doesn't mean you have low intelligence. "What it really says is that if you have a low IQ then there are high chances that you use Internet Explorer," said AptiQuant CEO Leonard Howard.
The findings have been treated with scepticism by Professor David Spiegelhalter of Cambridge University's Statistical Laboratory: "They've got IE6 users with an IQ of around eighty. That's borderline deficient, marginally able to cope with the adult world.
"I believe these figures are implausibly low - and an insult to IE users."
However, Mr Howard said he didn't feel threatened by a lawsuit: "A win in a court would only give a stamp of approval and more credibility to our report."