https://support.mozilla.org/en-US/questions/1020987#:~:text=Click%20the%20%E2%89%A1%20Menu%20Button,swatch%20and%20pick%20a%20color.

Hmm, it's a little complicated. If you want to inject a photo into the standard Firefox Home / new tab page, you need to get a little off the beaten track. If you want to explore a simpler option first, you could take a look at:

https://addons.mozilla.org/firefox/addon/new-tab-tools/

This thread has details on the style hack to modify the built-in page:

https://support.mozilla.org/en-US/questions/1293694#answer-1329980

its too complicated and take too much time and its not sure if it will work the i want.why must be this way? pls report it to the rigth division of firefox to make it easy or offer other way to add a pic as backgroud like google chrome does = very simple and nice .why firefox does not offer this way chrome does? end of the case ...