Hi Martin,
You could take the .jar theme of Orbit from either Mozilla, or Firefox (quite sure there's one for Mozilla Suite?) and unjar the file into a folder, search through the different directories for the icons you want and use those.
The problem with this is that the icons are in .png format usually and need to be converted over into .bmp, resized, and then "cleared up", and then they have to have the "magic pink" backgrounds added to them. (255, 0, 255 I believe the RGB is.) Then they're ready for use. Sounds like a chore but it's actually quite fun.
Just remember that if you choose to distribute the theme anywhere on the net to include a ReadMe.txt file in the .zip and give proper credit to all the icon makers, and to yourself as the editor. After all, you deserve some of the credit for adapting and porting, right?
Amicalement,
Eyes-Only
"L'Peau-Rouge"