--- a/toolkit/components/extensions/test/mochitest/test_ext_webrequestblocking_set_cookie.html
+++ b/toolkit/components/extensions/test/mochitest/test_ext_webrequestblocking_set_cookie.html
@@ -101,16 +101,45 @@ add_task(async function test_modifying_c
         };
       };
       browser.webRequest.onHeadersReceived.addListener(secondOnHeadersRecievedListiner, requestFilter, extraInfoSpec);
 
       await browser.browsingData.removeCookies({});
       const secondCookieSettingTabDetails = await openWindowAndTab("file_webrequestblocking_set_cookie.html");
       await checkCookies("ext", "req", "third");
       await browser.windows.remove(secondCookieSettingTabDetails.windowId);
+
+      // Fourth, test to make sure that extensions can remove cookies
+      // using onHeadersReceived too, by 1. making a request that
+      // sets a cookie (reqcookie=reqvalue), 2. having the extension remove
+      // that cookie by removing that header, and 3. adding a new cookie
+      // (extcookie=extvalue).
+      browser.webRequest.onHeadersReceived.removeListener(onHeadersReceived);
+      browser.webRequest.onHeadersReceived.removeListener(secondOnHeadersRecievedListiner);
+
+      const thirdOnHeadersRecievedListiner = details => {
+        // Remove the cookie set by the request (reqcookie=reqvalue).
+        const newHeaders = details.responseHeaders.filter(cookie => cookie.name !== "set-cookie");
+
+        // And then add a new cookie in its place (extcookie=extvalue).
+        newHeaders.push({
+          name: "Set-Cookie",
+          value: "extcookie=extvalue",
+        });
+
+        return {
+          responseHeaders: newHeaders,
+        };
+      };
+      browser.webRequest.onHeadersReceived.addListener(thirdOnHeadersRecievedListiner, requestFilter, extraInfoSpec);
+
+      await browser.browsingData.removeCookies({});
+      const thirdCookieSettingTabDetails = await openWindowAndTab("file_webrequestblocking_set_cookie.html");
+      await checkCookies("ext");
+      await browser.windows.remove(thirdCookieSettingTabDetails.windowId);
     }
 
     browser.test.notifyPass("cookie modifying extension");
   }
 
   extension = ExtensionTestUtils.loadExtension({
     manifest: {
       permissions: [