--- a/toolkit/components/extensions/ExtensionContent.jsm
+++ b/toolkit/components/extensions/ExtensionContent.jsm
@@ -719,16 +719,18 @@ class UserScript extends Script {
     }
 
     for (let key of Object.keys(userScriptAPIs)) {
       Schemas.exportLazyGetter(userScriptScope, key, () => {
         // Wrap the custom API methods exported to the userScript sandbox.
         return wrapUserScriptAPIMethod(userScriptAPIs[key], key);
       });
     }
+
+    context.userScriptsEvents.emit("on-before-script", clonedMetadata, userScriptScope);
   }
 }
 
 var contentScripts = new DefaultWeakMap(matcher => {
   const extension = processScript.extensions.get(matcher.extension);
 
   if ("userScriptOptions" in matcher) {
     return new UserScript(extension, matcher);
@@ -848,16 +850,21 @@ class ContentScriptContextChild extends 
 
     // A set of exported API methods provided by the extension to the userScripts sandboxes.
     this.userScriptAPIs = null;
 
     // Keep track if the userScript API script has been already executed in this context
     // (e.g. because there are more then one UserScripts that match the related webpage
     // and so the UserScript apiScript has already been executed).
     this.hasUserScriptAPIs = false;
+
+    // A lazy created EventEmitter related to userScripts-specific events.
+    defineLazyGetter(this, "userScriptsEvents", () => {
+      return new ExtensionCommon.EventEmitter();
+    });
   }
 
   injectAPI() {
     if (!this.isExtensionPage) {
       throw new Error("Cannot inject extension API into non-extension window");
     }
 
     // This is an iframe with content script API enabled (See Bug 1214658)

--- a/toolkit/components/extensions/child/ext-toolkit.js
+++ b/toolkit/components/extensions/child/ext-toolkit.js
@@ -72,16 +72,17 @@ extensions.registerModules({
       ["userScripts"],
     ],
   },
   userScriptsContent: {
     url: "chrome://extensions/content/child/ext-userScripts-content.js",
     scopes: ["content_child"],
     paths: [
       ["userScripts", "setScriptAPIs"],
+      ["userScripts", "onBeforeScript"],
     ],
   },
   webRequest: {
     url: "chrome://extensions/content/child/ext-webRequest.js",
     scopes: ["addon_child"],
     paths: [
       ["webRequest"],
     ],

--- a/toolkit/components/extensions/child/ext-userScripts-content.js
+++ b/toolkit/components/extensions/child/ext-userScripts-content.js
@@ -17,12 +17,30 @@ this.userScriptsContent = class extends 
       userScripts: {
         setScriptAPIs(exportedAPIMethods) {
           if (!userScriptsEnabled) {
             throw new ExtensionError(USERSCRIPT_DISABLED_ERRORMSG);
           }
 
           context.setUserScriptAPIs(exportedAPIMethods);
         },
+        onBeforeScript: new EventManager({
+          context,
+          name: "userScripts.onBeforeScript",
+          register: fire => {
+            let handler = (event, userScriptMetadata, userScriptSandbox) => {
+              const apiObj = Cu.createObjectIn(context.cloneScope);
+              apiObj.metadata = userScriptMetadata;
+              apiObj.global = userScriptSandbox;
+
+              fire.raw(apiObj);
+            };
+
+            context.userScriptsEvents.on("on-before-script", handler);
+            return () => {
+              context.userScriptsEvents.off("on-before-script", handler);
+            };
+          },
+        }).api(),
       },
     };
   }
 };

--- a/toolkit/components/extensions/ext-toolkit.json
+++ b/toolkit/components/extensions/ext-toolkit.json
@@ -196,17 +196,18 @@
     "paths": [
       ["userScripts"]
     ]
   },
   "userScriptsContent": {
     "schema": "chrome://extensions/content/schemas/user_scripts_content.json",
     "scopes": ["content_child"],
     "paths": [
-      ["userScripts", "setScriptAPIs"]
+      ["userScripts", "setScriptAPIs"],
+      ["userScripts", "onBeforeScript"]
     ]
   },
   "webNavigation": {
     "url": "chrome://extensions/content/parent/ext-webNavigation.js",
     "schema": "chrome://extensions/content/schemas/web_navigation.json",
     "scopes": ["addon_parent"],
     "paths": [
       ["webNavigation"]

--- a/toolkit/components/extensions/schemas/user_scripts_content.json
+++ b/toolkit/components/extensions/schemas/user_scripts_content.json
@@ -24,11 +24,36 @@
         "description": "Provides a set of custom API methods available to the registered userScripts",
         "parameters": [
           {
             "name": "exportedAPIMethods",
             "$ref": "ExportedAPIMethods"
           }
         ]
       }
+    ],
+    "events": [
+      {
+        "name": "onBeforeScript",
+        "permissions": ["manifest:user_scripts.api_script"],
+        "allowedContexts": ["content", "content_only"],
+        "type": "function",
+        "description": "Event called when a new userScript global has been created",
+        "parameters": [
+          {
+            "type": "object",
+            "name": "userScript",
+            "properties": {
+              "metadata": {
+                "type": "any",
+                "description": "The userScript metadata (as set in userScripts.register)"
+              },
+              "global": {
+                "type": "any",
+                "description": "The userScript global"
+              }
+            }
+          }
+        ]
+      }
     ]
   }
 ]