--- a/toolkit/components/extensions/test/mochitest/head_webrequest.js
+++ b/toolkit/components/extensions/test/mochitest/head_webrequest.js
@@ -227,17 +227,21 @@ function background(events) {
         // sure they're all consistent.
         expectedIp = expectedIp || details.ip;
         browser.test.assertEq(expectedIp, details.ip, `correct ip for ${details.url}`);
       }
       if (expected.headers && expected.headers.response) {
         checkHeaders("response", expected, details);
       }
     },
-    onErrorOccurred() {},
+    onErrorOccurred(expected, details, result) {
+      if (expected.error) {
+        browser.test.assertEq(expected.error, details.error, "expected error message received in onErrorOccurred");
+      }
+    },
   };
 
   function getListener(name) {
     return details => {
       let result = {};
       browser.test.log(`${name} ${details.requestId} ${details.url}`);
       let expected = getExpected(details);
       if (!expected) {

--- a/toolkit/components/extensions/test/mochitest/test_ext_webrequest_basic.html
+++ b/toolkit/components/extensions/test/mochitest/test_ext_webrequest_basic.html
@@ -344,16 +344,17 @@ add_task(async function test_webRequest_
     "dummy_page.html": {
       type: "sub_frame",
       status: 404,
     },
     "badrobot": {
       type: "sub_frame",
       status: 404,
       events: ["onBeforeRequest", "onBeforeSendHeaders", "onSendHeaders", "onErrorOccurred"],
+      error: "NS_ERROR_UNKNOWN_HOST",
     },
   };
   extension.sendMessage("set-expected", {expect, origin: location.href});
   await extension.awaitMessage("continue");
   addFrame("data:text/plain,webRequestTest");
   addFrame("data:text/plain,webRequestTest_bad");
   await extension.awaitMessage("cancelled");
   addFrame("redirection.sjs");