Firefox 5 has been released. I have installed it on my Macbook. I haven't got it on Linux yet, as it hasn't come down the pipe from Slackware packages.
The Mac version looks the same, pretty much. I don't like the default layout much, so I changed a few things. I get rid of the bookmark toolbar. That's a waste of space. And I see that Plus-Sign button to create a new tab has moved up to the navigation toolbar. I don't like it there. I want it down with the tab list, where it is perfectly ergonomically positioned to create a new tab. Every new release of Firefox is several big steps ahead in terms of technical excellence, and one step forward and one step backwards in the user interface. When they clean it up, a new version comes out and the UI gets irritating again.
Addendum: I spent some time fiddling with the interface. I right clicked on the tab bar, and selected customise. Then I grabbed the "New Tab" button from up on the navigation bar and dragged it down and dropped it on the tab bar. It took. A small plus sign appeared at the far right of the tab bar. After I dismissed the customise window, the plus sign moved over to just after the last tab and became a button, exactly what I wanted. I'm happy again.