diff --git a/js/xpconnect/tests/mochitest/finalizationGroup_worker.js b/js/xpconnect/tests/mochitest/finalizationGroup_worker.js
new file mode 100644
index 000000000000..fddf68a480a9
--- /dev/null
+++ b/js/xpconnect/tests/mochitest/finalizationGroup_worker.js
@@ -0,0 +1,96 @@
+let holdings1 = [];
+let holdings2 = [];
+let holdings3 = [];
+let holdings4 = [];
+let holdings5 = [];
+
+onmessage = (event) => {
+  switch (event.data) {
+  case 'startTest':
+    startTest();
+    break;
+  case 'checkResults':
+    checkResults();
+    break;
+  default:
+    throw "Unknown message";
+  }
+};
+
+function startTest() {
+  // Group with no registered objects.
+  let group1 = new FinalizationGroup(i => holdings1 = [...i]);
+
+  // Group with three registered objects.
+  let group2 = new FinalizationGroup(i => holdings2 = [...i]);
+  group2.register({}, 1);
+  group2.register({}, 2);
+  group2.register({}, 3);
+
+  // Group with registered object that is then unregistered.
+  let group3 = new FinalizationGroup(i => holdings3 = [...i]);
+  let token3 = {}
+  group3.register({}, 1, token3);
+  group3.unregister(token3);
+
+  // Group with registered object that doesn't die.
+  let group4 = new FinalizationGroup(i => holdings4 = [...i]);
+  let object4 = {};
+  group4.register(object4, 1);
+
+  // Group observing cyclic JS data structure.
+  let group5 = new FinalizationGroup(i => holdings5 = [...i]);
+  group5.register(makeJSCycle(4), 5);
+
+  const { gc } = getJSTestingFunctions();
+  gc();
+
+  Promise.resolve().then(() => {
+    checkNoCallbacks();
+  });
+
+  postMessage('started');
+}
+
+function checkNoCallbacks() {
+  is(holdings1.length, 0);
+  is(holdings2.length, 0);
+  is(holdings3.length, 0);
+  is(holdings4.length, 0);
+  is(holdings5.length, 0);
+}
+
+function checkResults() {
+  is(holdings1.length, 0);
+
+  let result = holdings2.sort((a, b) => a - b);
+  is(result.length, 3);
+  is(result[0], 1);
+  is(result[1], 2);
+  is(result[2], 3);
+
+  is(holdings3.length, 0);
+  is(holdings4.length, 0);
+
+  is(holdings5.length, 1);
+  is(holdings5[0], 5);
+
+  postMessage('passed');
+}
+
+function is(a, b) {
+  if (a !== b) {
+    throw `Expected ${b} but got ${a}`;
+  }
+}
+
+function makeJSCycle(size) {
+  let first = {};
+  let current = first;
+  for (let i = 0; i < size; i++) {
+    current.next = {};
+    current = current.next;
+  }
+  current.next = first;
+  return first;
+}
diff --git a/js/xpconnect/tests/mochitest/mochitest.ini b/js/xpconnect/tests/mochitest/mochitest.ini
index db00d80b48ae..d57b3dceb8b5 100644
--- a/js/xpconnect/tests/mochitest/mochitest.ini
+++ b/js/xpconnect/tests/mochitest/mochitest.ini
@@ -36,6 +36,9 @@ support-files =
   inner.html
   test1_bug629331.html
   test2_bug629331.html
+  finalizationGroup_worker.js
+prefs =
+  javascript.options.experimental.weakrefs=true
 
 [test_bug384632.html]
 [test_bug390488.html]
@@ -110,3 +113,7 @@ skip-if = (debug == false)
   support-files =
     ../../../../dom/tests/mochitest/fetch/test_fetch_basic.js
 [test_weakmaps.html]
+[test_finalizationGroup.html]
+skip-if = !nightly_build
+[test_finalizationGroupInWorker.html]
+skip-if = !nightly_build
diff --git a/js/xpconnect/tests/mochitest/test_finalizationGroup.html b/js/xpconnect/tests/mochitest/test_finalizationGroup.html
new file mode 100644
index 000000000000..635bdbef1cf4
--- /dev/null
+++ b/js/xpconnect/tests/mochitest/test_finalizationGroup.html
@@ -0,0 +1,112 @@
+<!DOCTYPE HTML>
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>Test FinalizationGroup works in the browser</title>
+    <script src="/tests/SimpleTest/SimpleTest.js"></script>
+    <script type="application/javascript">
+      function go() {
+        SimpleTest.waitForExplicitFinish();
+
+        // Group with no registered objects.
+        let holdings1 = [];
+        let group1 = new FinalizationGroup(i => holdings1 = [...i]);
+
+        // Group with three registered objects.
+        let holdings2 = [];
+        let group2 = new FinalizationGroup(i => holdings2 = [...i]);
+        group2.register({}, 1);
+        group2.register({}, 2);
+        group2.register({}, 3);
+
+        // Group with registered object that is then unregistered.
+        let holdings3 = [];
+        let group3 = new FinalizationGroup(i => holdings3 = [...i]);
+        let token3 = {}
+        group3.register({}, 1, token3);
+        group3.unregister(token3);
+
+        // Group with registered object that doesn't die.
+        let holdings4 = [];
+        let group4 = new FinalizationGroup(i => holdings4 = [...i]);
+        let object4 = {};
+        group4.register(object4, 1);
+
+        // Group observing cyclic JS data structure.
+        let holdings5 = [];
+        let group5 = new FinalizationGroup(i => holdings5 = [...i]);
+        group5.register(makeJSCycle(4), 5);
+
+        // Group observing cyclic DOM/JS data structure.
+        let holdings6 = [];
+        let group6 = new FinalizationGroup(i => holdings6 = [...i]);
+        group6.register(makeDOMCycle(4), 6);
+
+        // Need to run full GC/CC/GC cycle to collect cyclic garbage through DOM
+        // and JS heaps.
+        SpecialPowers.DOMWindowUtils.garbageCollect();
+        SpecialPowers.DOMWindowUtils.cycleCollect();
+        SpecialPowers.DOMWindowUtils.garbageCollect();
+
+        // Microtasks are run before cleanup callbacks.
+        Promise.resolve().then(() => {
+          is(holdings1.length, 0);
+          is(holdings2.length, 0);
+          is(holdings3.length, 0);
+          is(holdings4.length, 0);
+          is(holdings5.length, 0);
+          is(holdings6.length, 0);
+        });
+
+        // setTimeout queues a task which will run after cleanup callbacks.
+        setTimeout(() => {
+          is(holdings1.length, 0);
+
+          let result = holdings2.sort((a, b) => a - b);
+          is(result.length, 3);
+          is(result[0], 1);
+          is(result[1], 2);
+          is(result[2], 3);
+
+          is(holdings3.length, 0);
+          is(holdings4.length, 0);
+
+          is(holdings5.length, 1);
+          is(holdings5[0], 5);
+
+          is(holdings6.length, 1);
+          is(holdings6[0], 6);
+
+          SimpleTest.finish();
+        }, 0);
+      }
+
+      function makeJSCycle(size) {
+        let first = {};
+        let current = first;
+        for (let i = 0; i < size; i++) {
+          current.next = {};
+          current = current.next;
+        }
+        current.next = first;
+        return first;
+      }
+
+      function makeDOMCycle(size) {
+        let first = {};
+        let current = first;
+        for (let i = 0; i < size; i++) {
+          if (i % 2 === 0) {
+            current.next = document.createElement("div");
+          } else {
+            current.next = {};
+          }
+          current = current.next;
+        }
+        current.next = first;
+        return first;
+      }
+    </script>
+  </head>
+  <body onload="go()"></body>
+</html>
diff --git a/js/xpconnect/tests/mochitest/test_finalizationGroupInWorker.html b/js/xpconnect/tests/mochitest/test_finalizationGroupInWorker.html
new file mode 100644
index 000000000000..551dd2e011d4
--- /dev/null
+++ b/js/xpconnect/tests/mochitest/test_finalizationGroupInWorker.html
@@ -0,0 +1,40 @@
+<!DOCTYPE HTML>
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>Test FinalizationGroup works in workers</title>
+    <script src="/tests/SimpleTest/SimpleTest.js"></script>
+    <script type="application/javascript">
+      function go() {
+        SimpleTest.waitForExplicitFinish();
+
+        let worker = new Worker('finalizationGroup_worker.js');
+
+        worker.onevent = (event) => {
+          console.log(event.message);
+          throw event.error;
+        };
+
+        worker.onmessage = (event) => {
+          switch (event.data) {
+          case 'started':
+            worker.postMessage('checkResults');
+            break;
+
+          case 'passed':
+            ok(true, "Tests passed");
+            SimpleTest.finish();
+            break;
+
+          default:
+            console.log(event.data);
+            break;
+          }
+        };
+
+        worker.postMessage('startTest');
+      }
+    </script>
+  </head>
+  <body onload="go()"></body>
+</html>
