Assuming you only want bookmarks and related settings, this should work.
{
"$schema": "http://json-schema.org/schema#",
"title": "Mozilla Firefox (org.mozilla.Firefox)",
"description": "Settings for Mozilla Firefox",
"definitions": {
"bookmark": {"type": "object", "title": "Bookmark", "properties": {"Folder": {"type": "string", "title": "Folder"}, "URL": {"type": "string", "title": "URL"}, "Title": {"type": "string", "title": "Title"}, "Favicon": {"type": "string", "title": "Favicon"}, "Placement": {"type": "string", "title": "Placement", "options": {"enum_titles": ["toolbar", "menu"]}, "enum": ["toolbar", "menu"], "required": ["URL","Title"]}}}
},
"type": "object",
"properties": {
"Bookmarks": { "anyOf": [ {"type": "null", "title": "Not Configured"}, {"title": "Configured", "type": "array", "items": {"$ref": "#/definitions/bookmark"}, "default": [ {"Folder":"Example1Folder","URL":"https://www.example.org","Title":"Example1","Favicon":"https://www.example.org/favicon.ico","Placement":"toolbar"}]}]},
"DisplayBookmarksToolbar": {"title": "Display Bookmarks Toolbar", "description": "Set the initial state of the bookmarks toolbar. A user can still hide it and it will stay hidden.", "anyOf":[{"type":"null","title":"Not Configured"},{"title":"Configured", "type": "boolean", "default": "true"}]},
"NoDefaultBookmarks": {"title": "No Default Bookmarks", "description": "Disable the creation of default bookmarks. This policy is only effective if the user profile has not been created yet.", "anyOf":[{"type":"null","title":"Not Configured"},{"title":"Configured", "type": "boolean", "default": "true"}]}
}
}
(Edit) Fixed my typos. I was copying/pasting out of my full json for it and overlooked a few brackets and commas.
