Download the latest version of Firefox, install it (Install the DOM Inspector as well).
Extract the Theme
While you can hypotheticaly begin with any theme already designed for firefox, for the sake of consistency we'll speak as though everyone is editing Winstripe (the default firefox theme). This is located in the file "classic.jar" found in the firefox instalation directory. A .jar file is in reality a renamed zip and most zip and you can open them up in your zip program the same way you open up zip files. However if you zip aplication doesn't think it's of the right format, you can simply rename the file "classic.zip" and continue extraction.
Windows: <probably program files somewhere>
Mac OS X: I have no idea
Copy classic.jar to another easily accessible folder, Classic, is recommended, and extract the contents of that folder, maintaining directory structure.