Currently firefox doesn't allow me to get to the point of creating a theme background as well (but could I be the one who hasn't succeeded yet); On the web though there are many backgrounds with cubes that you can add in firefox (definitely from version 130 onwards, maybe even earlier). Here I'll explain: On the top right of the firefox homepage there's a little gear icon. Click there and you can change the background to the homepage.
Read more: https://support.mozilla.org/en-US/kb/customize-your-wallpapers

Yes of course you can modify it. Permission is granted already implicitly in the license. You can edit by writing in the description the source and non-profit.

Very kind, thank you

And in case you upgrade to the Noble Numbat version look for my theme in line with the official grape colored wallpaper of Ubuntu 24.04 LTS from Canonical:

https://ubuntu.com/blog/the-coronation-of-a-new-mascot-noble-numbat

You can also configure the Windows taskbar to be one with the Coronation Wallpaper and my Ubuntu Noble LTS firefox theme. Using the rgb(103, 24, 71) color palette or HEX #671847.

See this: https://www.w3schools.com/colors/colors_rgb.asp

Alles ist schön auf der Welt, großartige Landschaften, aber Europa ist mein Land. Wir haben alle Landschaften der Welt schon hier in Europa, man muss sie nur suchen: von Dünen über große Klippen bis zu Seen und majestätischen Wäldern, von aktiven Vulkanen über Berge bis zu großen Meeren. Und was der Mensch hier gemacht hat, ist immer noch majestätisch!

All beautiful the world, great landscapes, but Europe is my land. We have all the landscapes of the world already here in Europe, just look for them: from dunes to great cliffs to majestic lakes and forests, from active volcanoes to mountains to great seas. And what humans have done here is still majestic! I'm Italian.

Thanks for your review!