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.
Inside Classic.jar you will find 4 main directories
Browser contains all the toolbar icons, as well as the icons for the bookmark manager and the preferences window.
Global contains almost all of the important css files that define the apearance of the browser. This is the most critical directory in a theme.
Mozapps contains all the styles and icons for the browser periphrials, such as the extension manager or update wizard.
Doesn't do a whole lot and can typicaly be forgotten about promptly.