Download of Firefox 5 is already available.
The new Firefox 5 is here. Mozilla did not yet publish the Firefox 5 announcement on their site, but 9to5Mac published the direct download links for Mac OS X, Windows and Linux here.
I already installed Firefox 5 on Mac OS X. I have it running since a couple of hours and it seems to behave better than FF 4 in terms of memory and CPU usage. Besides performance there are several other improvements found in Firefox 5 including CSS animation, HTML5 and more.
Mozilla accelerated the release schedule for Firefox. The beta of Firefox 5 just launched a couple weeks ago and Firefox 6 is on the way.
I have now settled using three browsers at the same time for work. Firefox, Safari and Chrome. I have Firefox and Chrome side by side and Safari is on a its own expose desktop. I started using Chrome just recently as FF 4 hogged the system after a couple of hours when having two Windows open. Maybe with Firefox 5 I could get rid of Chrome again, but I do not have time to test this.