--- a/browser/components/originattributes/test/browser/browser_firstPartyIsolation_aboutPages.js
+++ b/browser/components/originattributes/test/browser/browser_firstPartyIsolation_aboutPages.js
@@ -7,43 +7,43 @@ add_task(async function setup() {
 });
 
 /**
  * For loading the initial about:blank in e10s mode, it will be loaded with
  * NullPrincipal, and we also test if the firstPartyDomain of the origin
  * attributes is got from the origin itself.
  */
 add_task(async function test_remote_window_open_aboutBlank() {
-  let win = await BrowserTestUtils.openNewBrowserWindow({ remote: true });
+  let win = await BrowserTestUtils.openNewBrowserWindow({ remote: true }, "about:blank");
   let browser = win.gBrowser.selectedBrowser;
 
   Assert.ok(browser.isRemoteBrowser, "should be a remote browser");
 
-  await ContentTask.spawn(browser, {}, async function() {
+  let attrs = { firstPartyDomain: "about.ef2a7dd5-93bc-417f-a698-142c3116864f.mozilla" };
+  await ContentTask.spawn(browser, attrs, async function(expectedAttrs) {
     info("origin " + content.document.nodePrincipal.origin);
 
     Assert.ok(content.document.nodePrincipal.isNullPrincipal,
               "The principal of remote about:blank should be a NullPrincipal.");
 
-    let str = content.document.nodePrincipal.originNoSuffix;
-    let expectDomain = str.substring("moz-nullprincipal:{".length, str.length - 1) + ".mozilla";
+    let expectDomain = expectedAttrs.firstPartyDomain;
     Assert.equal(content.document.nodePrincipal.originAttributes.firstPartyDomain,
                  expectDomain,
                  "remote about:blank should have firstPartyDomain set to " + expectDomain);
   });
 
   win.close();
 });
 
 /**
  * For loading the initial about:blank in non-e10s mode, it will be loaded with
  * a null principal. So we test if it has correct firstPartyDomain set.
  */
 add_task(async function test_nonremote_window_open_aboutBlank() {
-  let win = await BrowserTestUtils.openNewBrowserWindow({remote: false});
+  let win = await BrowserTestUtils.openNewBrowserWindow({remote: false}, "about:blank");
   let browser = win.gBrowser.selectedBrowser;
 
   Assert.ok(!browser.isRemoteBrowser, "shouldn't be a remote browser");
 
   let attrs = { firstPartyDomain: "about.ef2a7dd5-93bc-417f-a698-142c3116864f.mozilla" };
   await ContentTask.spawn(browser, attrs, async function(expectAttrs) {
     Assert.ok(!content.document.nodePrincipal.isCodebasePrincipal,
               "The principal of non-remote about:blank should not be a codebase principal.");
@@ -68,17 +68,17 @@ function frame_script() {
  * Check if data: URI inherits firstPartyDomain from about:blank correctly.
  */
 add_task(async function test_remote_window_open_data_uri() {
   // allow top level data: URI navigations, otherwise
   // <a href="data:" would fail.
   await SpecialPowers.pushPrefEnv({
     "set": [["security.data_uri.block_toplevel_data_uri_navigations", false]]
   });
-  let win = await BrowserTestUtils.openNewBrowserWindow({ remote: true });
+  let win = await BrowserTestUtils.openNewBrowserWindow({ remote: true }, "about:blank");
   let browser = win.gBrowser.selectedBrowser;
   let mm = browser.messageManager;
   mm.loadFrameScript("data:,(" + frame_script.toString() + ")();", true);
 
   await BrowserTestUtils.browserLoaded(browser, false, function(url) {
     return url == "data:text/plain,hello";
   });
 
@@ -95,17 +95,17 @@ add_task(async function test_remote_wind
   win.close();
 });
 
 /**
  * data: document contains an iframe, and we test that iframe should inherit
  * origin attributes from the data: document.
  */
 add_task(async function test_remote_window_open_data_uri2() {
-  let win = await BrowserTestUtils.openNewBrowserWindow({ remote: true });
+  let win = await BrowserTestUtils.openNewBrowserWindow({ remote: true }, "about:blank");
   let browser = win.gBrowser.selectedBrowser;
 
   // The iframe test2.html will fetch test2.js, which will have cookies.
   const DATA_URI = `data:text/html,
                     <iframe id="iframe1" src="http://mochi.test:8888/browser/browser/components/originattributes/test/browser/test2.html"></iframe>`;
   browser.loadURI(DATA_URI);
   await BrowserTestUtils.browserLoaded(browser, true);
 

--- a/browser/components/originattributes/test/browser/browser_firstPartyIsolation_js_uri.js
+++ b/browser/components/originattributes/test/browser/browser_firstPartyIsolation_js_uri.js
@@ -2,40 +2,42 @@ add_task(async function setup() {
   Services.prefs.setBoolPref("privacy.firstparty.isolate", true);
 
   registerCleanupFunction(function() {
     Services.prefs.clearUserPref("privacy.firstparty.isolate");
   });
 });
 
 add_task(async function test_remote_window_open_js_uri() {
-  let win = await BrowserTestUtils.openNewBrowserWindow({ remote: true });
+  // Load a new browser window without any document
+  let win = await BrowserTestUtils.openNewBrowserWindow({ remote: true }, "");
   let browser = win.gBrowser.selectedBrowser;
 
   Assert.ok(browser.isRemoteBrowser, "should be a remote browser");
 
   browser.loadURI(`javascript:1;`);
   await ContentTask.spawn(browser, {}, async function() {
     info("origin " + content.document.nodePrincipal.origin);
 
     Assert.ok(content.document.nodePrincipal.isNullPrincipal,
               "The principal of remote javascript: should be a NullPrincipal.");
 
-    let str = content.document.nodePrincipal.originNoSuffix;
-    let expectDomain = str.substring("moz-nullprincipal:{".length, str.length - 1) + ".mozilla";
+    // Since there is no document loaded into the <browser>, the domain is empty
+    let expectDomain = "";
     Assert.equal(content.document.nodePrincipal.originAttributes.firstPartyDomain,
                  expectDomain,
                  "remote javascript: should have firstPartyDomain set to " + expectDomain);
   });
 
   win.close();
 });
 
 add_task(async function test_remote_window_open_js_uri2() {
-  let win = await BrowserTestUtils.openNewBrowserWindow({ remote: true });
+  // Load a new browser window with a custom document
+  let win = await BrowserTestUtils.openNewBrowserWindow({ remote: true }, "data:text/html,<html><?html>");
   let browser = win.gBrowser.selectedBrowser;
 
   Assert.ok(browser.isRemoteBrowser, "should be a remote browser");
 
   browser.loadURI(`javascript:
     let iframe = document.createElement("iframe");
     iframe.src = "http://example.com";
     iframe.id = "iframe1";
@@ -49,18 +51,18 @@ add_task(async function test_remote_wind
   });
 
   await ContentTask.spawn(browser, {}, async function() {
     info("origin " + content.document.nodePrincipal.origin);
 
     Assert.ok(content.document.nodePrincipal.isNullPrincipal,
               "The principal of remote javascript: should be a NullPrincipal.");
 
-    let str = content.document.nodePrincipal.originNoSuffix;
-    let expectDomain = str.substring("moz-nullprincipal:{".length, str.length - 1) + ".mozilla";
+    // Since there is no document loaded into the <browser>, the domain is empty
+    let expectDomain = "";
     Assert.equal(content.document.nodePrincipal.originAttributes.firstPartyDomain,
                  expectDomain,
                  "remote javascript: should have firstPartyDomain set to " + expectDomain);
 
     let iframe = content.document.getElementById("iframe1");
     Assert.equal(iframe.contentDocument.nodePrincipal.originAttributes.firstPartyDomain,
                  expectDomain,
                  "iframe should have firstPartyDomain set to " + expectDomain);