There are more than 250 themes available for Firefox. You can find all of them here. If you want to “Christianize” your browsing experience, I recommend that you use the Bible Fox theme. As you can see in the screenshot, the icons used in this theme are familiar Christian symbols (fish, dove, cross, church, etc). This theme is available for Windows, Mac and Linux. It is compatible with Firefox 3.0.
Ubuntu Christian Edition users do not have to install this skin because Bible Fox is the default Firefox theme of their distro. It might interest you to know that this theme averages more than 8,000 downloads per week.