Firefox should have the native ability (as do all the Chromium browsers - i.e. Chrome, Chromium, Edge, Opera, Brave, etc.) to be able to add a image (whether custom from your own PC or changing from a website) to the Firefox Home New Tab page to be used as wallpaper. Yes, there is the ability to install some extensions to do this, but many people prefer to just use the built-in browser functionality for simplicity and/or security reasons. This isn't a new question, it has been asked before, but the solution was simply a hack which involved adding directories, files and changing about:config settings which many people aren't capable or comfortable of doing. Not to mention the workaround didn't provide the full functionality for Chromium browser feature parity that is being requested. I'm a bit surprised that this issue wasn't addressed with Photon. Being an issue of feature parity, it's kind of a big miss.