The first Release Candidate for Firefox 4 is now available for download. People who already use the existing beta will receive the update automatically.It's available for Windows, Linux and Mac users.
There is an extensive list of bugs that are fixed in RC1, and Mozilla claims that this build will provide stability, performance and compatibility improvements. It's also claiming that 70% of Firefox add-ons will now work with Firefox 4.
Since switching back to Firefox 4 I've had no problems at all and every site I've been on has worked entirely as expected. It will be interesting to see how the other browsers fare against Firefox 4 when all of the latest versions are released.