I prefer a Mac OS X styled theme, but any would be fine as long as it work on my Firefox 4 for Mac OS X.
2 Answers
You can find a lot of OS X-like themes in the OS Integration section of the Firefox Themes website. Each theme page will be able to tell you whether or not the theme is compatible with FF4.
In Firefox 4 on Mac OS X 10.6.7, FF4 by defaults disables the themes that are not compatbile with Firefox4 (In thumbnail mode). The above OS Integration URL has many themes.
No need of going to each theme page.