I downloaded the Firefox 4 beta today, and I am impressed. It addresses many of my grievances that I had with Firefox 3 regarding speed.
Firefox 3 was slooooooooooooooooooooooooooooow. It took hours to start up compared to Chrome, the "awesome bar" would often hang while loading the bookmarks database into memory (making it not live up to its name), and there would be constant "small freezes" here and there while loading PDF files and such where I wouldn't be able to use my browser for a second or two because something is loading.
It looks like Firefox 4 has made all of that asynchronous (as it should be). From my experience so far, I am still able to use the browser while certain things load, and the awesome bar now lives up to its name (not hanging while loading the entire bookmarks database).
Have yall downloaded it and tried it out?
There's still a few seconds wait time when first starting up the browser, but it seems less than Firefox 3.