I have been an avid user of Firefox for a very long time, certainly ever since Netscape became as bad as IE quite a few years back. However I feel that the new release of Firefox is a step in the wrong direction. I like the new look and I am getting used to the new interface, but I really dislike something that has been done to the underlying code somewhere. Reskinning ones browser is all well and good; albeit that the update to 4.0 made Firefox less distinctive than it was before.
The big grief here is that sites are taking longer to load with the new version of Firefox than they had just a day before. Hopefully this gets corrected with the next update as it is really unacceptable. I also hope that the visual look of Firefox 4.0 get some tweaks to make it look... well more like a Firefox browser of some kind. The cleaner interface is nice, but it would be good to have a little more color or the Firefox logo in more than just the uppermost left corner of the window.
All in all I do not feel as though the change to Firefox 4.0 was an upgrade. Loss of speed and a step away from being distinctive are not good directions.