For Ubuntu users, there's the Firefox Aurora channel. Firefox builds currently in the Aurora channel are meant for eventual release as Firefox 6. Here's how to get in on the action now, for the impatient:
~$ sudo add-apt-repository ppa:ubuntu-mozilla-daily/firefox-aurora
~$ sudo apt-get update
~$ sudo apt-get install firefox
With that, you should be running Firefox Aurora in short order.
|Firefox Aurora aka "Firefox 6 Alpha 2"|
Now, I think I'll see how much faster Aurora is compared to Firefox 4...