Style Mozilla applications the way you want with pixel-perfect themes.
Learn how to find the right files to edit, make your changes, and package up your new theme in this tutorial.
Thomas McMahon explains how to create a Firefox theme from start-to-finish in this tutorial.
This article explains the packaging of themes.
This blog post by Lim Chee Aun explains the use of -moz-image-region in themes.
It's important to make sure your theme works in all locales. This article explains how to tweak your theme to look great for users who browse in right-to-left.
If you haven't found what you're looking for yet, try the Mozilla Developer Center's Themes section.