A Selection of the best themes avalaible for Firefox. Includes IE, Chrome, Safari, Opera and Flock Update: Unfortunately I've been unable to edit this collection. So... https://.../addon/australis/
Killer Add-ons with extended/specific features for power users. If you are looking for themes or security addons you may then check Killer Themes or Security & Privacy Compilation.
The best Security & Privacy Add-ons around for Firefox + Mcafee SiteAdvisor (not included in AMO)