It's not all about Mozilla's pride, really! Firefox 3 will bring many changes for all Mac users (mostly good), as this is the first release of Firefox that is built primarily using Cocoa for the underlying Gecko 1.9 engine (a project that's been in the works for years). Along with better redraw speed (CoreGraphics vs. the older QuickDraw API) and improved memory handling, the new engine provides the groundwork for a pure-Cocoa, 64-bit clean version of Firefox in the future. Firefox 3's UI is more Maclike now, with Aqua-style widgets improving the view. You can read more about the architectural changes to Firefox 3 and Gecko 1.9 at developer Josh Aas' blog.
So head on over to Spread Firefox, we're sure you can't go wrong! If you're in the mood to party with the Firefox launch day crowd, check Mozilla's party site for activities near you.