As you can see, Mozilla has began working on the Multi-OS Interface for Firefox.
Its Currently in Firefox 3 Beta 3 but is disabled. This is a picture of it enabled and in Windows Vista.
Mozilla is doing a good job at redesigning the UI especially for multi-OSes. From what I know so far. There isn't a way to force enable the Mac OS X UI or even enable it on a Mac OS X system at this time, But By Firefox 3 Beta 4 (final build of beta 4). It should be enabled automatically.
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