It's going to be in a number of different files depending on what specific things you want to change globally. I think you can use dconf-editor to set some globals for desktop things.
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I think you want a skeleton. /etc/skel
If you want to copy settings for Firefox and other applications, copy (parts of) your .config in there. When you create a new user it uses the skel directory as a template.
Google/GPT will explain it better than I can.
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