Because it's been requested so often, I've created a special page for generating a version of Fierr with a custom background image. Simply select the images and colors you want to use and a custom extension will be created for you to install. Any fields you do not fill out will be replaced with Fierr defaults.
Please note that this extension will not be digitally signed, and will not work on the latest versions of Firefox. You need to disable the extension signing requirement to use it. The easiest way to do this is to use a development version of Firefox instead of the standard Release version.
No support is provided for customized extensions. You're on your own with anything you create here.