diff --git a/packages/browser/src/transports/new-xhr.ts b/packages/browser/src/transports/new-xhr.ts
index 7373b946a505..7f9cad6cdfa1 100644
--- a/packages/browser/src/transports/new-xhr.ts
+++ b/packages/browser/src/transports/new-xhr.ts
@@ -7,9 +7,17 @@ import {
 } from '@sentry/core';
 import { SyncPromise } from '@sentry/utils';
 
+/**
+ * The DONE ready state for XmlHttpRequest
+ *
+ * Defining it here as a constant b/c XMLHttpRequest.DONE is not always defined
+ * (e.g. during testing, it is `undefined`)
+ *
+ * @see {@link https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/readyState}
+ */
+const XHR_READYSTATE_DONE = 4;
+
 export interface XHRTransportOptions extends BaseTransportOptions {
-  // TODO choose whatever is preferred here (I like record more for easier readability)
-  //headers?: { [key: string]: string };
   headers?: Record<string, string>;
 }
 
@@ -22,8 +30,7 @@ export function makeNewXHRTransport(options: XHRTransportOptions): NewTransport
       const xhr = new XMLHttpRequest();
 
       xhr.onreadystatechange = (): void => {
-        // TODO make 4 a constant
-        if (xhr.readyState === 4) {
+        if (xhr.readyState === XHR_READYSTATE_DONE) {
           const response = {
             body: xhr.response,
             headers: {
@@ -33,17 +40,18 @@ export function makeNewXHRTransport(options: XHRTransportOptions): NewTransport
             reason: xhr.statusText,
             statusCode: xhr.status,
           };
-
           resolve(response);
         }
       };
 
       xhr.open('POST', options.url);
+
       for (const header in options.headers) {
         if (Object.prototype.hasOwnProperty.call(options.headers, header)) {
           xhr.setRequestHeader(header, options.headers[header]);
         }
       }
+
       xhr.send(request.body);
     });
   }
diff --git a/packages/browser/test/unit/transports/new-xhr.test.ts b/packages/browser/test/unit/transports/new-xhr.test.ts
index 4deee14f3d46..a7c67e7b2396 100644
--- a/packages/browser/test/unit/transports/new-xhr.test.ts
+++ b/packages/browser/test/unit/transports/new-xhr.test.ts
@@ -21,7 +21,7 @@ function createXHRMock() {
     status: 200,
     response: 'Hello World!',
     onreadystatechange: () => {},
-    getResponseHeader: (header: string) => {
+    getResponseHeader: jest.fn((header: string) => {
       switch (header) {
         case 'Retry-After':
           return '10';
@@ -30,7 +30,7 @@ function createXHRMock() {
         default:
           return `${retryAfterSeconds}:error:scope`;
       }
-    },
+    }),
   };
 
   // casting `window` as `any` because XMLHttpRequest is missing in Window (TS-only)
@@ -56,16 +56,53 @@ describe('NewXHRTransport', () => {
     expect(xhrMock.setRequestHeader).toHaveBeenCalledTimes(0);
     expect(xhrMock.send).toHaveBeenCalledTimes(0);
 
+    await Promise.all([transport.send(ERROR_ENVELOPE), (xhrMock as XMLHttpRequest).onreadystatechange(null)]);
+
+    expect(xhrMock.open).toHaveBeenCalledTimes(1);
+    expect(xhrMock.open).toHaveBeenCalledWith('POST', DEFAULT_XHR_TRANSPORT_OPTIONS.url);
+    expect(xhrMock.send).toHaveBeenCalledTimes(1);
+    expect(xhrMock.send).toHaveBeenCalledWith(serializeEnvelope(ERROR_ENVELOPE));
+  });
+
+  it('returns the correct response', async () => {
+    const transport = makeNewXHRTransport(DEFAULT_XHR_TRANSPORT_OPTIONS);
+
     const [res] = await Promise.all([
       transport.send(ERROR_ENVELOPE),
       (xhrMock as XMLHttpRequest).onreadystatechange(null),
     ]);
 
-    expect(xhrMock.open).toHaveBeenCalledTimes(1);
-    expect(xhrMock.open).toHaveBeenCalledWith('POST', DEFAULT_XHR_TRANSPORT_OPTIONS.url);
-    expect(xhrMock.send).toBeCalledWith(serializeEnvelope(ERROR_ENVELOPE));
-
     expect(res).toBeDefined();
     expect(res.status).toEqual('success');
   });
+
+  it('sets rate limit response headers', async () => {
+    const transport = makeNewXHRTransport(DEFAULT_XHR_TRANSPORT_OPTIONS);
+
+    await Promise.all([transport.send(ERROR_ENVELOPE), (xhrMock as XMLHttpRequest).onreadystatechange(null)]);
+
+    expect(xhrMock.getResponseHeader).toHaveBeenCalledTimes(2);
+    expect(xhrMock.getResponseHeader).toHaveBeenCalledWith('X-Sentry-Rate-Limits');
+    expect(xhrMock.getResponseHeader).toHaveBeenCalledWith('Retry-After');
+  });
+
+  it('sets custom request headers', async () => {
+    const headers = {
+      referrerPolicy: 'strict-origin',
+      keepalive: 'true',
+      referrer: 'http://example.org',
+    };
+    const options: XHRTransportOptions = {
+      ...DEFAULT_XHR_TRANSPORT_OPTIONS,
+      headers,
+    };
+
+    const transport = makeNewXHRTransport(options);
+    await Promise.all([transport.send(ERROR_ENVELOPE), (xhrMock as XMLHttpRequest).onreadystatechange(null)]);
+
+    expect(xhrMock.setRequestHeader).toHaveBeenCalledTimes(3);
+    expect(xhrMock.setRequestHeader).toHaveBeenCalledWith('referrerPolicy', headers.referrerPolicy);
+    expect(xhrMock.setRequestHeader).toHaveBeenCalledWith('keepalive', headers.keepalive);
+    expect(xhrMock.setRequestHeader).toHaveBeenCalledWith('referrer', headers.referrer);
+  });
 });