diff --git a/testing/mochitest/browser-test.js b/testing/mochitest/browser-test.js
index 262ce8334c75..8ba4411b9c33 100644
--- a/testing/mochitest/browser-test.js
+++ b/testing/mochitest/browser-test.js
@@ -338,6 +338,10 @@ Tester.prototype = {
       // Thunderbird
       "MailMigrator",
       "SearchIntegration",
+      // lit
+      "reactiveElementVersions",
+      "litHtmlVersions",
+      "litElementVersions",
     ];
 
     this.PerTestCoverageUtils.beforeTestSync();
diff --git a/toolkit/content/tests/widgets/test_moz_button_group.html b/toolkit/content/tests/widgets/test_moz_button_group.html
index 88418f58c398..73727c195364 100644
--- a/toolkit/content/tests/widgets/test_moz_button_group.html
+++ b/toolkit/content/tests/widgets/test_moz_button_group.html
@@ -32,22 +32,43 @@
       let beforeButton = document.getElementById("before-button");
       let afterButton = document.getElementById("after-button");
 
-      async function checkButtons(first, second) {
-        let firstBounds = first.getBoundingClientRect();
-        let secondBounds = second.getBoundingClientRect();
+      async function checkButtons(...buttons) {
+        const checkOrder = (a, b) => {
+          let firstBounds = a.getBoundingClientRect();
+          let secondBounds = b.getBoundingClientRect();
 
-        ok(firstBounds.x < secondBounds.x, `First button comes first`);
-        let locationDiff = Math.abs(secondBounds.x - firstBounds.x - firstBounds.width - 8);
-        ok(locationDiff < 1, `Second button is 8px after first (${locationDiff})`);
+          ok(firstBounds.x < secondBounds.x, `First button comes first`);
+          let locationDiff = Math.abs(secondBounds.x - firstBounds.x - firstBounds.width - 8);
+          ok(locationDiff < 1, `Second button is 8px after first (${locationDiff})`);
+        };
 
+        ok(buttons.length >= 2, "There are at least 2 buttons to check");
+
+        // Verify tab order is correct.
         beforeButton.focus();
         is(document.activeElement, beforeButton, "Before button is focused");
+
         synthesizeKey("VK_TAB");
-        is(document.activeElement, first, "First button is first in tab order");
-        synthesizeKey("VK_TAB");
-        is(document.activeElement, second, "Second button is next in tab order");
+        is(document.activeElement, buttons[0], "Next button is focused");
+
+        for (let i = 1; i < buttons.length; i++) {
+          // Confirm button order in DOM
+          checkOrder(buttons[i - 1], buttons[i]);
+
+          synthesizeKey("VK_TAB");
+          is(document.activeElement, buttons[i], "Next button is focused");
+        }
+
         synthesizeKey("VK_TAB");
         is(document.activeElement, afterButton, "After button is at the end in tab order");
+
+        // Verify light DOM order is correct, in case of manual tab management in JS.
+        let { parentElement } = buttons[0];
+        let parentChildren = parentElement.children;
+        is(parentChildren.length, buttons.length, "Expected number of children");
+        for (let i = 0; i < parentChildren.length; i++) {
+          is(parentChildren[i], buttons[i], `Button ${i} is in correct light DOM spot`);
+        }
       }
 
 
@@ -151,6 +172,65 @@
         await buttonGroup.updateComplete;
         await checkButtons(secondaryButton, autofocusButton);
       });
+
+      add_task(async function testDefaultButtonAutoSlotting() {
+        render(
+          html`
+            <moz-button-group>
+              <button default>First</button>
+              <button class="secondary">Secondary</button>
+            </moz-button-group>
+          `,
+          renderArea
+        );
+
+        let buttonGroup = document.querySelector("moz-button-group");
+        let defaultButton = buttonGroup.querySelector("[default]");
+        let secondaryButton = buttonGroup.querySelector(".secondary");
+        buttonGroup.platform = "win";
+        await buttonGroup.updateComplete;
+        is(defaultButton.slot, "primary", "default button was auto-slotted")
+        await checkButtons(defaultButton, secondaryButton);
+
+        buttonGroup.platform = "macosx";
+        await buttonGroup.updateComplete;
+        await checkButtons(secondaryButton, defaultButton);
+      });
+
+      add_task(async function testInitialButtonLightDomReordering() {
+        const renderPlatform = platform => render(
+          html`
+            <moz-button-group .platform=${platform}>
+              <button class="primary">First</button>
+              <button class="secondary">Secondary</button>
+              <button default>Default</button>
+            </moz-button-group>
+          `,
+          renderArea
+        );
+
+        renderPlatform("win");
+        let buttonGroup = document.querySelector("moz-button-group");
+        await buttonGroup.updateComplete;
+        let primaryButton = buttonGroup.querySelector(".primary");
+        let defaultButton = buttonGroup.querySelector("[default]");
+        let secondaryButton = buttonGroup.querySelector(".secondary");
+
+        is(primaryButton.slot, "primary", "primary button was auto-slotted");
+        is(defaultButton.slot, "primary", "default button was auto-slotted");
+        await checkButtons(primaryButton, defaultButton, secondaryButton);
+
+        renderPlatform("macosx");
+        buttonGroup = document.querySelector("moz-button-group");
+        await buttonGroup.updateComplete;
+        primaryButton = buttonGroup.querySelector(".primary");
+        defaultButton = buttonGroup.querySelector("[default]");
+        secondaryButton = buttonGroup.querySelector(".secondary");
+
+        is(primaryButton.slot, "primary", "primary button was auto-slotted");
+        is(defaultButton.slot, "primary", "default button was auto-slotted");
+        await checkButtons(secondaryButton, primaryButton, defaultButton);
+      });
     </script>
   </body>
 </html>
diff --git a/toolkit/content/widgets/moz-button-group/moz-button-group.mjs b/toolkit/content/widgets/moz-button-group/moz-button-group.mjs
index 0d98bb94dfb8..1ee0a6b28aa1 100644
--- a/toolkit/content/widgets/moz-button-group/moz-button-group.mjs
+++ b/toolkit/content/widgets/moz-button-group/moz-button-group.mjs
@@ -60,11 +60,34 @@ export default class MozButtonGroup extends MozLitElement {
         child.localName == "button" &&
         (child.classList.contains("primary") ||
           child.getAttribute("type") == "submit" ||
-          child.hasAttribute("autofocus"))
+          child.hasAttribute("autofocus") ||
+          child.hasAttribute("default"))
       ) {
         child.slot = "primary";
       }
     }
+    this.#reorderLightDom();
+  }
+
+  #reorderLightDom() {
+    let primarySlottedChildren = [...this.primarySlotEl.assignedNodes()];
+    if (this.platform == PLATFORM_WINDOWS) {
+      primarySlottedChildren.reverse();
+      for (let child of primarySlottedChildren) {
+        child.parentElement.prepend(child);
+      }
+    } else {
+      for (let child of primarySlottedChildren) {
+        // Ensure the primary buttons are at the end of the light DOM.
+        child.parentElement.append(child);
+      }
+    }
+  }
+
+  updated(changedProperties) {
+    if (changedProperties.has("platform")) {
+      this.#reorderLightDom();
+    }
   }
 
   render() {