I regularly use Firefox 6 and Chrome on my home computer (WinXP). On my MacBook Pro (OS 10.4) I use Safari, Chrome, and Firefox. On my work PC (Windows 7) I use IE, Chrome, Safari, and Firefox. Aside from the odd compatibility quirk here and there, all of these browsers feel exactly the same speed to me. The Internet connection is the biggest factor in speed followed by the number of add-ons where available. No matter what I do, the daily use of all of these browsers on all these platforms yields my complete confusion as to how anyone can tell a difference in speed between any of them. I'm sure there is a technical difference in speed, but I often forget which one I'm using unless I specifically look towards the corners of the browser window and see the UI. The experiences are largely the same.
I can understand different C/GPU's and the like because of performance but I'm puzzled about browsers. Why is browser speed such a big deal?
fwiw, I find it useful to have all of these options so I can load multiple instances of a web page, for example, in different browsers, or have different browsers for different logins.