Other than another potential delay, Firefox 4 Beta 12 arrived a couple days ago with little more than stability, performance and compatibility improvements. Check out the release notes. It sounds minor but those are the three most important pillars of any software these days.
There's 11 blocker bugs left at time of posting this according to canweshipyet.com but the numbers keep bouncing up and down. I wouldn't be overly reliant on that counter.
As usual there's always the daily PPA for Ubuntu users wanting to give the latest build a go. Keep in mind the PPA contains pre-Beta builds. That's currently pre-Beta 13 but it could get released as RC depending on how things go.
Installing Firefox 4 pre-Beta 8 on Ubuntu 10.10
Firefox 4 Beta 10 is here
Four versions of Firefox in a year is insane