Firefox 3.0's new default Mac theme showed up today in the current trunk nightly (Minefield). The theme is very similar to Safari's:

Most dialogs and other windows (e.g., Places and History) also have refined UI elements.

I've noticed a few issues in the short amount of time I've had to play with it (this is completely expected for a nightly build). The toolbar buttons freak out every once in a while and stack up on each other. The browser also looks a little funny when you have the bookmarks toolbar turned off - the tab colors don't go smoothly into the address bar.

The theme has been available as an add-on since the second beta but this should make it official.