I am really impressed. I was testing my Fullerscreen extension on Firefox 3.0b1 and tried to show my last ParisWeb2007 slides. These slides are a good test case for me because they include an SVG-based background. And I let go a rather sonic "wow!" when I saw FF3 switch almost instantanenously from the alert window "This document appears to be a slideshow. Do you want to switch to slideshow mode?" to the first slide... Between those two, the extension takes a screenshot of every slide into a canvas element to create slide thumbnails. On FF2, that takes time, a lot of time, when there's an SVG background. So much time I just added a "creating thumbnails, please wait" message. On FF3, that's amazingly fast...

There's more : shift-F11 raises the slide manager with all thumbnails. Navigating between thumbnails using the keyboard was slow too, and my CPU fan was going nuts. It's now super-fast and my CPU fan remains quiet.

This is far more interesting to me than the UI improvements because it clears the path for highly graphic consuming  web or XUL apps. This is also the kind of things reviewers do not notice when they "review" the new version of a browser.  FF3 will kick ass !