--- a/toolkit/components/extensions/ext-c-test.js
+++ b/toolkit/components/extensions/ext-c-test.js
@@ -35,14 +35,41 @@ function testApiFactory(context) {
           actual += " (different)";
         }
         context.childManager.callParentFunctionNoReturn("test.assertEq", [
           expected,
           actual,
           String(msg),
         ]);
       },
+
+      assertNoLastError(message) {
+        let {lastError} = context;
+        context.childManager.callParentFunctionNoReturn("test.assertEq", [
+          null,
+          lastError ? "lastError.message: " + lastError.message : lastError,
+          message,
+        ]);
+      },
+
+      assertLastError(expectedError) {
+        let {lastError} = context;
+        let actualError = "<lastError not set>";
+        if (lastError) {
+          let {message} = lastError;
+          if (typeof message == "string") {
+            actualError = message;
+          } else {
+            actualError = "(NOT A STRING) " + message;
+          }
+        }
+        context.childManager.callParentFunctionNoReturn("test.assertEq", [
+          expectedError,
+          actualError,
+          "Expected lastError",
+        ]);
+      },
     },
   };
 }
 extensions.registerSchemaAPI("test", "addon_child", testApiFactory);
 extensions.registerSchemaAPI("test", "content_child", testApiFactory);
 

--- a/toolkit/components/extensions/schemas/test.json
+++ b/toolkit/components/extensions/schemas/test.json
@@ -109,23 +109,23 @@
           {"type": "any", "name": "expected", "optional": true},
           {"type": "any", "name": "actual", "optional": true},
           {"type": "string", "name": "message", "optional": true}
         ]
       },
       {
         "name": "assertNoLastError",
         "type": "function",
-        "unsupported": true,
-        "parameters": []
+        "parameters": [
+          {"type": "string", "name": "message"}
+        ]
       },
       {
         "name": "assertLastError",
         "type": "function",
-        "unsupported": true,
         "parameters": [
           {"type": "string", "name": "expectedError"}
         ]
       },
       {
         "name": "assertThrows",
         "type": "function",
         "unsupported": true,

--- a/toolkit/components/extensions/test/mochitest/test_ext_test.html
+++ b/toolkit/components/extensions/test/mochitest/test_ext_test.html
@@ -49,16 +49,18 @@ function testScript() {
   browser.test.notifyPass("dot notifyPass");
   browser.test.notifyFail("dot notifyFail");
   browser.test.log("dot log");
   browser.test.fail("dot fail");
   browser.test.succeed("dot succeed");
   browser.test.assertTrue(true);
   browser.test.assertFalse(false);
   browser.test.assertEq("", "");
+  browser.test.assertNoLastError("dot assertNoLastError");
+  browser.test.assertLastError("dot assertLastError");
 
   let obj = {};
   let arr = [];
   let dom = document.createElement("body");
   browser.test.assertTrue(obj, "Object truthy");
   browser.test.assertTrue(arr, "Array truthy");
   browser.test.assertTrue(dom, "Element truthy");
   browser.test.assertTrue(true, "True truthy");
@@ -103,16 +105,18 @@ function verifyTestResults(results, shor
     ["test-done", true, "dot notifyPass"],
     ["test-done", false, "dot notifyFail"],
     ["test-log", true, "dot log"],
     ["test-result", false, "dot fail"],
     ["test-result", true, "dot succeed"],
     ["test-result", true, "undefined"],
     ["test-result", true, "undefined"],
     ["test-eq", true, "undefined", "", ""],
+    ["test-eq", true, "dot assertNoLastError", "null", "null"],
+    ["test-eq", false, "Expected lastError", "dot assertLastError", "<lastError not set>"],
 
     ["test-result", true, "Object truthy"],
     ["test-result", true, "Array truthy"],
     ["test-result", true, "Element truthy"],
     ["test-result", true, "True truthy"],
     ["test-result", false, "False truthy"],
     ["test-result", false, "Null truthy"],
     ["test-result", false, "Void truthy"],