gecko-dev/extensions/permissions/test/browser_permmgr_viewsrc.js

add_task(async function() {
  // Add a permission for example.com, start a new content process, and make
  // sure that the permission has been sent down.
  Services.perms.add(
    Services.io.newURI("http://example.com"),
    "viewsourceTestingPerm",
    Services.perms.ALLOW_ACTION
  );

  let tab = await BrowserTestUtils.openNewForegroundTab(
    gBrowser,
    "view-source:http://example.com",
    /* waitForLoad */ true,
    /* waitForStateStop */ false,
    /* forceNewProcess */ true
  );
  await ContentTask.spawn(tab.linkedBrowser, null, async function() {
    is(
      Services.perms.testPermission(
        Services.io.newURI("http://example.com"),
        "viewsourceTestingPerm"
      ),
      Services.perms.ALLOW_ACTION
    );
  });
  BrowserTestUtils.removeTab(tab);
});