Can't stop firefox from changing desktop background

[ZSH]

I set all the group policies I could find to stop students from changing their background.

All is fine with windows explorer and Internet explorer, users can't change their background.

But if they use Firefox then they can right click any image within a web page and "set as desktop background" which will bypass all group policy and set it as a background, only until next log on.

Any help how to stop it?

 

Thanks