It looks like Firefox 4 is going to have some very nice features, but they might be old news if you are an Apple fanboy.
To download Firefox 4 Beta (codenamed ‘Minefield’ hehe), go to this link.
Then, click on the OS version you are running (I clicked on tryserver-win32), and then download and run the appropriate ‘installer’ package.
I finished this post in the new version, so it seems to do the basics ok!