I noticed something odd the other day with the google results. My main keyword is listed at #6 in IE, but for Firefox its listed at #3. Im not logged into Gmail for both browsers also.
The browsers have different cookies. Check to make sure "Web History" (in the upper right of the serps page) is disabled... and then also clear your cookies, and see if the results are closer.
Other factors might include your "Location" setting (in left column), and what data center and combination of data sets you happen to get when you search.
