I haven't tried Firefox mobile in the last month or so, but I did use it briefly for a while. I found the native browser to be much better. FF would take a couple seconds to reload a section of page once you moved off it. I found that very disconcerting, especially when the native browser doesn't have to do that.
I really like Firefox and have used it for years on my PC, but the mobile app seems to have a way to go yet, at least for Froyo 2.2