devtools.panels.ExtensionSidebarPane.setPage()

extensionPageURL

string. The relative path of an HTML page to display within the sidebar.

A Promise that will be fulfilled with no arguments, once the URL has been set.

The selected page will not be loaded until the user selects the devtools sidebar.

Create a new pane, and populate it with an HTML page. You could run this code in a script loaded by your extension's devtools page.

function onCreated(sidebarPane) {
  sidebarPane.setPage('sidebar/sidebar.html');
}