Hmmm. Why will Google generate different search results for different browsers? Isn't that pointless? I'll research on this though. And then make my educated guess.
IE and FF shall have same results if your browser settings have the same num, lang, location. They are same results.
There is no conspiracy theory, but yes sometimes the results are different in different browsers, as they connect to different datacenters even when you type the same google.com. In this particular case I believe that there are the FF settings that override standard request. It is to long to explain, but if you want to know more just find the way to check from which IP you get the answer when you type google.com.
Thank you that might be the explanation for this, but strange that they are unable to manage it better, since Google is looking for providing the most accurate search result, then it makes no sense if the result is different when you use one or another browser.