Having said that I tried FF4 and now have tried FF5 for a couple of days. It seems a decent enough browser, but by comparisson the Chrome and IE9 it "feels" a bit slow and flabby. I'm not using any add-ons, and perhaps that's the bit i'm missing. If I relied on noscript and a bunch of other add-ons perhaps FF would be on the button. As it is it strikes me as a middle ground between IE9 and Chrome with no particular advantage over either.
Except perhaps the add-ons, and as I don't really use them maybe that's why FF5 has been a bit of a let down for me.