I couldn't find any thread mentioning this so I'm writing my own. I'm constantly getting different Google ranking for the same keyword in Internet Explorer and Firefox. While my main keyword gets to No. 12 (second page) in IE, I'm No.4 in Firefox! Now, before you start saying that it could be different browser preferences, cookies, Google preferences, whatever - I checked this on 5 different computers, both PCs and Macs. Same results. Plenty of my keywords are on the first page in Firefox. Same keywords appear on pages 2-3 in IE. Can anyone explain that? Gee, I wish everyone was using Firefox!
I have the exact same ranking for a bunch of my sites in both FF and IE. Maybe your browsers are getting results from different DCs and/or your listing is currently in a volatile state.
Are you signed in to Google at the time? An odd thing i found tonight is when searching "SEO Company" with the standard 10 results per page im around #5 which is normal, but.. Changing to 100 results per page i have a double listing with a sub page as well in positions 5 and 6. Strange, i never use 100 results so ill have to look in to it some more.
Search results may depend on what datacenter you use. For example, you might get different results for the same keyword if you search google.de compared to google.com. I think that in Firefox you're redirected to your country's datacenter, but I am not sure.
It has nothing to do with the browser...Your pulling from different Data centers and maybe dancing a little as well.