I'm sure they will fix the tab style issues in OS X before release:
"OSX and Linux Themes: The new interface described above will soon be available for Mac and Linux users."
At the bottom of: http://www.mozilla.com/en-US/firefox/beta/features/ note, "The new interface described above will soon be available for Mac and Linux users."
They also need to adopt something like Safari's URL-field-as-progress-indicator, or at least an animating graphic to indicate loading. Simply switching the refresh button to an 'X' is way too subtle, and at first I didn't think it was actually loading anything.
Edit: Oh, that's weird. I fired it up again and saw the stopwatch graphic and other loading indicators. Now I'm wondering if these didn't work the first time around, or if I just wasn't paying attention to them given that I'm habituated to Safari's indicators.