diff --git a/files/en-us/mozilla/add-ons/webextensions/api/runtime/index.md b/files/en-us/mozilla/add-ons/webextensions/api/runtime/index.md
index c1193e53e3aa231..0dea473d18b807b 100644
--- a/files/en-us/mozilla/add-ons/webextensions/api/runtime/index.md
+++ b/files/en-us/mozilla/add-ons/webextensions/api/runtime/index.md
@@ -31,6 +31,10 @@ It also provides messaging APIs enabling you to:
   - : Result of a call to {{WebExtAPIRef("runtime.requestUpdateCheck()")}}.
 - {{WebExtAPIRef("runtime.OnInstalledReason")}}
   - : The reason that the {{WebExtAPIRef("runtime.onInstalled")}} event is being dispatched.
+- {{WebExtAPIRef("runtime.OnPerformanceWarningCategory")}}
+  - : The category of warning that dispatched the {{WebExtAPIRef("runtime.onPerformanceWarning")}} event.
+- {{WebExtAPIRef("runtime.OnPerformanceWarningSeverity")}}
+  - : The severity of warning that dispatched the {{WebExtAPIRef("runtime.onPerformanceWarning")}} event.
 - {{WebExtAPIRef("runtime.OnRestartRequiredReason")}}
   - : The reason that the {{WebExtAPIRef("runtime.onRestartRequired")}} event is being dispatched.
 
@@ -96,6 +100,8 @@ It also provides messaging APIs enabling you to:
   - : Fired when a message is sent from either an extension process or a content script.
 - {{WebExtAPIRef("runtime.onMessageExternal")}}
   - : Fired when a message is sent from another extension. Cannot be used in a content script.
+- {{WebExtAPIRef("runtime.onPerformanceWarning")}}
+  - : Fired when a runtime performance issue is detected for the extension.
 - {{WebExtAPIRef("runtime.onRestartRequired")}}
   - : Fired when the device needs to be restarted.
 
diff --git a/files/en-us/mozilla/add-ons/webextensions/api/runtime/onperformancewarning/index.md b/files/en-us/mozilla/add-ons/webextensions/api/runtime/onperformancewarning/index.md
new file mode 100644
index 000000000000000..3a57739329f6b35
--- /dev/null
+++ b/files/en-us/mozilla/add-ons/webextensions/api/runtime/onperformancewarning/index.md
@@ -0,0 +1,64 @@
+---
+title: runtime.onPerformanceWarning
+slug: Mozilla/Add-ons/WebExtensions/API/runtime/onPerformanceWarning
+page-type: webextension-api-event
+browser-compat: webextensions.api.runtime.onPerformanceWarning
+---
+
+{{AddonSidebar}}
+
+This event fires when a runtime performance issue is detected for the extension. Observe this event to be notified of runtime performance problems with your extension.
+
+## Syntax
+
+```js-nolint
+browser.runtime.onPerformanceWarning.addListener(listener)
+browser.runtime.onPerformanceWarning.removeListener(listener)
+browser.runtime.onPerformanceWarning.hasListener(listener)
+```
+
+Events have three functions:
+
+- `addListener(listener)`
+  - : Adds a listener to this event.
+- `removeListener(listener)`
+  - : Stop listening to this event. The `listener` argument is the listener to remove.
+- `hasListener(listener)`
+  - : Checks whether at least one listener is registered for this event. Returns `true` if it is listening, `false` otherwise.
+
+## addListener syntax
+
+### Parameters
+
+- `listener`
+
+  - : The function called when this event occurs. The function is passed this argument:
+
+    - `details`
+
+      - : `object`. An object with the following properties:
+
+        - `category`
+          - : {{WebExtAPIRef("runtime.OnPerformanceWarningCategory")}}. The category of the warning.
+        - `severity`
+          - : {{WebExtAPIRef("runtime.OnPerformanceWarningSeverity")}}. The severity of the warning.
+        - `tabId` {{optional_inline}}
+          - : `integer`. The ID of the tab that the performance warning relates to, if any.
+        - `description`
+          - : `string`. An explanation of what the warning means, possibly with information on how to address it.
+
+## Examples
+
+```js
+function handlePerformanceWarning(details) {
+  console.log(`Performance warning: ${details.description}`);
+}
+
+browser.runtime.onPerformanceWarning.addListener(handlePerformanceWarning);
+```
+
+{{WebExtExamples}}
+
+## Browser compatibility
+
+{{Compat}}
diff --git a/files/en-us/mozilla/add-ons/webextensions/api/runtime/onperformancewarningcategory/index.md b/files/en-us/mozilla/add-ons/webextensions/api/runtime/onperformancewarningcategory/index.md
new file mode 100644
index 000000000000000..b74c01acdfbd761
--- /dev/null
+++ b/files/en-us/mozilla/add-ons/webextensions/api/runtime/onperformancewarningcategory/index.md
@@ -0,0 +1,22 @@
+---
+title: runtime.OnPerformanceWarningCategory
+slug: Mozilla/Add-ons/WebExtensions/API/runtime/OnPerformanceWarningCategory
+page-type: webextension-api-type
+browser-compat: webextensions.api.runtime.OnPerformanceWarningCategory
+---
+
+{{AddonSidebar}}
+
+The category of warning that dispatched the {{WebExtAPIRef("runtime.onPerformanceWarning")}} event.
+
+## Type
+
+Values of this type are strings. Possible values are:
+
+- `"content_script"`: The performance warning is for a slow content script in the listening extension.
+
+{{WebExtExamples("h2")}}
+
+## Browser compatibility
+
+{{Compat}}
diff --git a/files/en-us/mozilla/add-ons/webextensions/api/runtime/onperformancewarningseverity/index.md b/files/en-us/mozilla/add-ons/webextensions/api/runtime/onperformancewarningseverity/index.md
new file mode 100644
index 000000000000000..caf9f8afb7a5418
--- /dev/null
+++ b/files/en-us/mozilla/add-ons/webextensions/api/runtime/onperformancewarningseverity/index.md
@@ -0,0 +1,24 @@
+---
+title: runtime.OnPerformanceWarningSeverity
+slug: Mozilla/Add-ons/WebExtensions/API/runtime/OnPerformanceWarningSeverity
+page-type: webextension-api-type
+browser-compat: webextensions.api.runtime.OnPerformanceWarningSeverity
+---
+
+{{AddonSidebar}}
+
+The severity of warning that dispatched the {{WebExtAPIRef("runtime.onPerformanceWarning")}} event.
+
+## Type
+
+Values of this type are strings. Possible values are:
+
+- `"low"`
+- `"medium"`
+- `"high"`
+
+{{WebExtExamples("h2")}}
+
+## Browser compatibility
+
+{{Compat}}
diff --git a/files/en-us/mozilla/firefox/releases/124/index.md b/files/en-us/mozilla/firefox/releases/124/index.md
index b6afbbe48f70336..c26e0b096753b0d 100644
--- a/files/en-us/mozilla/firefox/releases/124/index.md
+++ b/files/en-us/mozilla/firefox/releases/124/index.md
@@ -58,6 +58,8 @@ This article provides information about the changes in Firefox 124 that affect d
 
 ## Changes for add-on developers
 
+- Adds the {{WebExtAPIRef("runtime.onPerformanceWarning")}} event that enables extensions to obtain information when the browser detects that the extension has a runtime performance issue such as a slow-running content script ([Firefox bug 1861445](https://bugzil.la/1861445)).
+
 ### Removals
 
 ### Other