Alex Faaborg, Mozilla's interface designer, posted a note on his blog yesterday about the divergent themes coming for Firefox 3. Echoing the sentiments of many a Mac Firefox user, Faaborg writes:

You may be detecting a trend. Visual integration with Windows and OS X is our primary objective for the Firefox 3 refresh. Fitting in to the visual appearance of the native operating system may seem like a reasonably obvious decision, but it certainly isn't one that every cross-platform application or windowing toolkit makes. For instance RealPlayer (image) uses a custom appearance across operating systems, as do applications built using Java's Swing windowing toolkit (image). Personally I think a unified cross platform UI results in applications that at best feel foreign everywhere, and at worst don't even feel like real applications. Making XUL appear truly native means we have to do some extra work when new operating systems are released, like Vista and Leopard, but I think this work clearly results in a superior user experience.

With this release of Firefox we are trying to achieve an even closer level of visual integration than we have had in previous releases. For instance, we are planning on having two different sets of icons for Windows: a set for Windows XP that uses the XP color palette, and will have the correct plastic reflectance, and a set for Windows Vista that will use a larger color pallet for objects, a smaller color pallet for glyphs, and the icons will be appropriately shiny. For Firefox 2 we weren't able to ship two icon sets, so we literally averaged the two different aesthetics and created a theme that looked sort of right on each platform. If you are interested in the design guidelines for Windows icons, they are available for XP and Vista, and here are the guidelines for different visual variables I've set up based on these documents for the new Firefox icons.