Ok I just wrote up a blog post here (http://internetbusinessdaily.net/google-changes-results-based-on-browser/) about this so I won't go into too much detail. Basically, Google displays two different search results pages at the same time from the same IP but on two different browsers. Safari is showing different results than Firefox. Very weird. I had no idea about this. Who knew??
Also if you have any Google account like Gmail etc open and different cookies on the browser will also affect the search results.. Google is playing around with the algorithms to try and give search results based on your history so pretty much different people will get tailored results even when searching the same search string / query.