Do you find yourself gazing upon Firefox's default bland-and-boring 'New Tab' page far more often that you do the wonderful wallpaper you oh so carefully chose to decorate your Desktop with? If so, then this is the add-on for you! Features

Custom background color.
Custom wallpaper image(s).
Custom redirection.
Custom top site visibility.

How to use local files as wallpapers Firefox currently does not allow WebExtensions to access 'file:///...' URLs. However, until that becomes possible, New Tab Tweaker does support the following workaround:

Install HTTP File Server (HFS)
Host your wallpaper directory on HFS.
Add the following line to the wallpaper URLs textbox in New Tab Tweaker's options (without quotes): '[list] http://localhost/[WP]/?tpl=list'. Where '[WP]' is the path to your wallpaper directory on HFS. If that directory contains sub-folders with images, replace '?tpl=list' with '?tpl=list&folders-filter=\&recursive' instead.

The URL above will instruct HFS to build a list of all files in your wallpaper directory. The [list] tag at the front of the line tells New Tab Tweaker to treat the URL not as an image, but as a list of URLs to images. One will be selected randomly from among them. You can configure HFS to launch at system start for your convenience. Something's not working? Got a question and/or suggestion? Post it here.