diff --git a/browser/components/extensions/test/browser/browser.toml b/browser/components/extensions/test/browser/browser.toml
index c219c4f17d656..31768424228ba 100644
--- a/browser/components/extensions/test/browser/browser.toml
+++ b/browser/components/extensions/test/browser/browser.toml
@@ -381,6 +381,8 @@ run-if = ["crashreporter"]
 
 ["browser_ext_request_permissions.js"]
 
+["browser_ext_runtime_onPerformanceWarning.js"]
+
 ["browser_ext_runtime_openOptionsPage.js"]
 
 ["browser_ext_runtime_openOptionsPage_uninstall.js"]
@@ -455,6 +457,7 @@ https_first_disabled = true
 skip-if = [
   "debug",
   "asan",
+  "tsan" # Bug 1874317
 ]
 
 ["browser_ext_tab_runtimeConnect.js"]
diff --git a/browser/components/extensions/test/browser/browser_ext_runtime_onPerformanceWarning.js b/browser/components/extensions/test/browser/browser_ext_runtime_onPerformanceWarning.js
new file mode 100644
index 0000000000000..b0f62e677a651
--- /dev/null
+++ b/browser/components/extensions/test/browser/browser_ext_runtime_onPerformanceWarning.js
@@ -0,0 +1,144 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+"use strict";
+
+const {
+  Management: {
+    global: { tabTracker },
+  },
+} = ChromeUtils.importESModule("resource://gre/modules/Extension.sys.mjs");
+
+const {
+  ExtensionUtils: { promiseObserved },
+} = ChromeUtils.importESModule("resource://gre/modules/ExtensionUtils.sys.mjs");
+
+class TestHangReport {
+  constructor(addonId, scriptBrowser) {
+    this.addonId = addonId;
+    this.scriptBrowser = scriptBrowser;
+    this.QueryInterface = ChromeUtils.generateQI(["nsIHangReport"]);
+  }
+
+  userCanceled() {}
+  terminateScript() {}
+
+  isReportForBrowserOrChildren(frameLoader) {
+    return (
+      !this.scriptBrowser || this.scriptBrowser.frameLoader === frameLoader
+    );
+  }
+}
+
+function dispatchHangReport(extensionId, scriptBrowser) {
+  const hangObserved = promiseObserved("process-hang-report");
+
+  Services.obs.notifyObservers(
+    new TestHangReport(extensionId, scriptBrowser),
+    "process-hang-report"
+  );
+
+  return hangObserved;
+}
+
+function background() {
+  let onPerformanceWarningDetails = null;
+
+  browser.runtime.onPerformanceWarning.addListener(details => {
+    onPerformanceWarningDetails = details;
+  });
+
+  browser.test.onMessage.addListener(message => {
+    if (message === "get-on-performance-warning-details") {
+      browser.test.sendMessage(
+        "on-performance-warning-details",
+        onPerformanceWarningDetails
+      );
+      onPerformanceWarningDetails = null;
+    }
+  });
+}
+
+async function expectOnPerformanceWarningDetails(
+  extension,
+  expectedOnPerformanceWarningDetails
+) {
+  extension.sendMessage("get-on-performance-warning-details");
+
+  let actualOnPerformanceWarningDetails = await extension.awaitMessage(
+    "on-performance-warning-details"
+  );
+  Assert.deepEqual(
+    actualOnPerformanceWarningDetails,
+    expectedOnPerformanceWarningDetails,
+    expectedOnPerformanceWarningDetails
+      ? "runtime.onPerformanceWarning fired with correct details"
+      : "runtime.onPerformanceWarning didn't fire"
+  );
+}
+
+add_task(async function test_should_fire_on_process_hang_report() {
+  const description =
+    "Slow extension content script caused a page hang, user was warned.";
+
+  const extension = ExtensionTestUtils.loadExtension({ background });
+  await extension.startup();
+
+  const notificationPromise = BrowserTestUtils.waitForGlobalNotificationBar(
+    window,
+    "process-hang"
+  );
+
+  const tabs = await Promise.all([
+    BrowserTestUtils.openNewForegroundTab(gBrowser),
+    BrowserTestUtils.openNewForegroundTab(gBrowser),
+  ]);
+
+  // Warning event shouldn't have fired initially.
+  await expectOnPerformanceWarningDetails(extension, null);
+
+  // Hang report fired for the extension and first tab. Warning event with first
+  // tab ID expected.
+  await dispatchHangReport(extension.id, tabs[0].linkedBrowser);
+  await expectOnPerformanceWarningDetails(extension, {
+    category: "content_script",
+    severity: "high",
+    description,
+    tabId: tabTracker.getId(tabs[0]),
+  });
+
+  // Hang report fired for different extension, no warning event expected.
+  await dispatchHangReport("wrong-addon-id", tabs[0].linkedBrowser);
+  await expectOnPerformanceWarningDetails(extension, null);
+
+  // Non-extension hang report fired, no warning event expected.
+  await dispatchHangReport(null, tabs[0].linkedBrowser);
+  await expectOnPerformanceWarningDetails(extension, null);
+
+  // Hang report fired for the extension and second tab. Warning event with
+  // second tab ID expected.
+  await dispatchHangReport(extension.id, tabs[1].linkedBrowser);
+  await expectOnPerformanceWarningDetails(extension, {
+    category: "content_script",
+    severity: "high",
+    description,
+    tabId: tabTracker.getId(tabs[1]),
+  });
+
+  // Hang report fired for the extension with no associated tab. Warning event
+  // with no tab ID expected.
+  await dispatchHangReport(extension.id, null);
+  await expectOnPerformanceWarningDetails(extension, {
+    category: "content_script",
+    severity: "high",
+    description,
+  });
+
+  await Promise.all(tabs.map(BrowserTestUtils.removeTab));
+  await extension.unload();
+
+  // Wait for the process-hang warning bar to be displayed, then ensure it's
+  // cleared to avoid clobbering other tests.
+  const notification = await notificationPromise;
+  Assert.ok(notification.isConnected, "Notification still present");
+  notification.buttonContainer.querySelector("[label='Stop']").click();
+});
diff --git a/toolkit/components/extensions/parent/ext-runtime.js b/toolkit/components/extensions/parent/ext-runtime.js
index ead070ac27f58..f4f9ea6616f29 100644
--- a/toolkit/components/extensions/parent/ext-runtime.js
+++ b/toolkit/components/extensions/parent/ext-runtime.js
@@ -4,6 +4,10 @@
 
 "use strict";
 
+// This file expects tabTracker to be defined in the global scope (e.g.
+// by ext-browser.js or ext-android.js).
+/* global tabTracker */
+
 var { ExtensionParent } = ChromeUtils.importESModule(
   "resource://gre/modules/ExtensionParent.sys.mjs"
 );
@@ -83,6 +87,43 @@ this.runtime = class extends ExtensionAPIPersistent {
         },
       };
     },
+    onPerformanceWarning({ fire }) {
+      let { extension } = this;
+
+      let observer = (subject, topic) => {
+        let report = subject.QueryInterface(Ci.nsIHangReport);
+
+        if (report?.addonId !== extension.id) {
+          return;
+        }
+
+        const performanceWarningEventDetails = {
+          category: "content_script",
+          severity: "high",
+          description:
+            "Slow extension content script caused a page hang, user was warned.",
+        };
+
+        let scriptBrowser = report.scriptBrowser;
+        let nativeTab =
+          scriptBrowser?.ownerGlobal.gBrowser?.getTabForBrowser(scriptBrowser);
+        if (nativeTab) {
+          performanceWarningEventDetails.tabId = tabTracker.getId(nativeTab);
+        }
+
+        fire.async(performanceWarningEventDetails);
+      };
+
+      Services.obs.addObserver(observer, "process-hang-report");
+      return {
+        unregister: () => {
+          Services.obs.removeObserver(observer, "process-hang-report");
+        },
+        convert(_fire, context) {
+          fire = _fire;
+        },
+      };
+    },
   };
 
   getAPI(context) {
@@ -171,6 +212,13 @@ this.runtime = class extends ExtensionAPIPersistent {
           },
         }).api(),
 
+        onPerformanceWarning: new EventManager({
+          context,
+          module: "runtime",
+          event: "onPerformanceWarning",
+          extensionApi: this,
+        }).api(),
+
         reload: async () => {
           if (extension.upgrade) {
             // If there is a pending update, install it now.
diff --git a/toolkit/components/extensions/schemas/runtime.json b/toolkit/components/extensions/schemas/runtime.json
index 5f894fa0e9740..84ef7d3ccbaf1 100644
--- a/toolkit/components/extensions/schemas/runtime.json
+++ b/toolkit/components/extensions/schemas/runtime.json
@@ -162,6 +162,18 @@
         "allowedContexts": ["content", "devtools"],
         "description": "The reason that the event is being dispatched. 'app_update' is used when the restart is needed because the application is updated to a newer version. 'os_update' is used when the restart is needed because the browser/OS is updated to a newer version. 'periodic' is used when the system runs for more than the permitted uptime set in the enterprise policy.",
         "enum": ["app_update", "os_update", "periodic"]
+      },
+      {
+        "id": "OnPerformanceWarningCategory",
+        "type": "string",
+        "enum": ["content_script"],
+        "description": "The performance warning event category, e.g. 'content_script'."
+      },
+      {
+        "id": "OnPerformanceWarningSeverity",
+        "type": "string",
+        "enum": ["low", "medium", "high"],
+        "description": "The performance warning event severity. Will be 'high' for serious and user-visible issues."
       }
     ],
     "properties": {
@@ -677,6 +689,36 @@
             "description": "The reason that the event is being dispatched."
           }
         ]
+      },
+      {
+        "name": "onPerformanceWarning",
+        "type": "function",
+        "description": "Fired when a runtime performance issue is detected with the extension. Observe this event to be proactively notified of runtime performance problems with the extension.",
+        "parameters": [
+          {
+            "type": "object",
+            "name": "details",
+            "properties": {
+              "category": {
+                "$ref": "OnPerformanceWarningCategory",
+                "description": "The performance warning event category, e.g. 'content_script'."
+              },
+              "severity": {
+                "$ref": "OnPerformanceWarningSeverity",
+                "description": "The performance warning event severity, e.g. 'high'."
+              },
+              "tabId": {
+                "type": "integer",
+                "optional": true,
+                "description": "The $(ref:tabs.Tab) that the performance warning relates to, if any."
+              },
+              "description": {
+                "type": "string",
+                "description": "An explanation of what the warning means, and hopefully how to address it."
+              }
+            }
+          }
+        ]
       }
     ]
   }
diff --git a/toolkit/components/extensions/test/mochitest/test_ext_all_apis.js b/toolkit/components/extensions/test/mochitest/test_ext_all_apis.js
index 4c82a4575cf83..95ac9af50df08 100644
--- a/toolkit/components/extensions/test/mochitest/test_ext_all_apis.js
+++ b/toolkit/components/extensions/test/mochitest/test_ext_all_apis.js
@@ -95,6 +95,7 @@ let expectedBackgroundApis = [
   "runtime.onConnectExternal",
   "runtime.onInstalled",
   "runtime.onMessageExternal",
+  "runtime.onPerformanceWarning",
   "runtime.onStartup",
   "runtime.onSuspend",
   "runtime.onSuspendCanceled",
@@ -102,6 +103,8 @@ let expectedBackgroundApis = [
   "runtime.openOptionsPage",
   "runtime.reload",
   "runtime.setUninstallURL",
+  "runtime.OnPerformanceWarningCategory",
+  "runtime.OnPerformanceWarningSeverity",
   "theme.getCurrent",
   "theme.onUpdated",
   "types.LevelOfControl",