bookmarks.getSubTree()

let gettingSubTree = browser.bookmarks.getSubTree(
  id                     // string
)

function makeIndent(indentLength) {
  return ".".repeat(indentLength);
}
function logItems(bookmarkItem, indent) {
  if (bookmarkItem.url) {
    console.log(makeIndent(indent) + bookmarkItem.url);
  } else {
    console.log(`${makeIndent(indent)}Folder: ${bookmarkItem.id}`);
    indent++;
  }
  if (bookmarkItem.children) {
    for (const child of bookmarkItem.children) {
      logItems(child, indent);
    }
  }
}
function logSubTree(bookmarkItems) {
  logItems(bookmarkItems[0], 0);
}
function onRejected(error) {
  console.log(`An error: ${error}`);
}
let subTreeID = "root_____";
browser.bookmarks.getSubTree(subTreeID).then(logSubTree, onRejected);