I just recently upgraded to Firefox 4. I am loving it so far. It's so fast and sleek. There is less hoop-la at the top of the window in the toolbar, so it feels like I have also upgraded to a wider screen. Ha! Of course there is room for upgrading the toolbar, but I like the feel of less buttons, and more viewing space.
The speed of Firefox 4 is great! I haven't hit a lag yet. Here's hoping for continued Sonic the Hedgehog-like speeds and success!