Just noticed this today for the first time. How can it be that two different browsers are taking data from two different datacenter's at the same time and showing different SERPs?
JackR, my experience is that the differences are much less than reported. I use Firefox but occasionally view sites in IE. If you use the same version of Google i.e. .com or .co.uk and use the same buttons you will get very similar results.
I posted a thread about this the other month, there is a difference with my SERPs too. We came to the conclusion it is based on datacentres as dburdon mentions.
I have heard that Google sometimes uses cookies when you do a search. The cookies in IE will be different from those in Firefox so you will have different results. Try clearing your cookies in both and see if the results come out the same.