Aeon Clouds works with Apple OS 10.6.7 and Firefox 4.0 Rated 5 out of 5 stars
iMac, OS 10.6.7
I installed FF 4.0 over FF 3.6.15 with Aeon Clouds as the theme. Aeon Clouds seems to be displaying absolutely normally in FF 4.0, despite all the warnings on this website. The Tools, Add-ons page says Aeon Clouds is incompatible, and the only option shown is to 'Remove' it. Don't do that unless you truly want to no longer use Aeon Clouds. If you do, the only way to get Aeon Cloud back into FF 4.0 is to re-install FF 3.6.15; add Aeon Clouds; and then upgrade to FF 4.0.
I contacted the author of this great theme, and he confirmed that the Windows version is not compatible with FF 4.0. So, Mac users apparently can continue using this theme.