They really should add the option to set custom one, especially on Android all of the currently available ones look so bad.
about:firefoxview#syncedtabs
takes you there if you have a way to set it as new tab.
It doesn’t let you set custom one at the moment, only those that you can see in the picture.
Send feedback to app developer
Android roadmap: https://developer.thunderbird.net/planning/android-roadmap
Out of all of them Iceraven has been my favorite simply because of its support for desktop extensions.
You can use more extensions on other ones as well:
Works on Firefox Beta, Firefox Nightly, Fennec and Mull.
It has 374 thumb ups here:
https://connect.mozilla.org/t5/ideas/wallpaper-in-new-tab/idi-p/172
And was suggested here multiple times:
https://connect.mozilla.org/t5/discussions/let-s-chat-about-the-firefox-new-tab-page/m-p/7632#U7632