Heres my collection of the Top 5 Firefox 3 Themes. Themes are judged on looks, user-friendliness and smooth running.

Note: All themes are for Firefox 3.0.1

1. IFox Metal:
   

   iFox Metal (Click to Enlarge)

   A nice clean Metallic theme which is soothing to the eyes, its rated 5/5!
   Download Here.
   

2. Noia 2.0 (eXtreme):
   


Noia 2.0 (eXtreme), (Click to Enlarge)

This is one of the best Firefox themes ever, based on a set of icons and version 3.0.1 has several bug fixs aswell as GUI fixs, it has clean glossy buttons and a silvery look to it.
Download Here.

3. Aero Silver Fox:
   


Aero Silver Fox (Click to Enlarge)

“Aero Fox Sliver redone for Firefox 3. Aero style glassy icons on a light background.. ”
Download Here.

4. Aquatint Black Gloss:
   

   Aquatint Black Gloss (Click to Enlarge)

   “Aquatint Black Gloss redone for Firefox 3. A glossy black theme with the icon set from Aquatint using glassy aqua buttons.. ”
   Download Here

5. Abstract Zune:
   

   Abstract Zune (Click to Enlarge)

   “Abstract Zune redone for Firefox 3. Glassy orange buttons in the style of Abstract to match the Windows Zune theme.”
   Download Here.

   Well these are the top 5 i chose i hope you like them! one last thing, to access currently installed themes go to Tools > Addons and Select Themes. Goodluck!