Well, Firefox 4b1 has just been lauched and we had the chance to test it and use it for quite a while.
The first thing I did was to test it running some crazy heacy scripts, like https://bit.ly/DvVL and https://bit.ly/ppahL. I gotta say, it was impressive. Those scripts simply couldn’t run at Firefox in the previous version, now, they run just as chrome does. It made me happy about this release.
Then, testing it with acid3, it reached the 97 points. I think it could be better, specially because either chrome, safari and opera reached 100. Though, those 3 missing points are related to the SVG Fonts, which were deliberately not implemented, using WOFF instead, as you can see in the Mozilla’s developers blog.
At html5test.com, firefox 4 had 189 points. Chrome was the only better, with 197. IE had 27, as I thought.
When testing selectors and CSS3 here, I notice there are only a few properties/methods that only chrome offers more than Firefox.
The problem I have to point by now is that no one single add-on worked on this version. Even mine didn’t work ok and I’ll have to change some details at them.

About the layout, I’m personaly happy they didn’t take off the status bar, as many people had asked for(if I wanted a browser without a status bar, I’d rather use Chrome). Still, if you use windows vista or Seven, you may have some new advances, like the special button at the top of the window, removing the usual menubar, adding it to the title bar.

I do think there are many things to get better or to be increased in Firefox 4 before its release candidate. It promises a lot and I’m really glad to see the browsers walking toward the same point. The future developers will be happier and faster :)