diff --git a/components/msg/constellation_msg.rs b/components/msg/constellation_msg.rs
index 6f1acd219cf..570e9e1c684 100644
--- a/components/msg/constellation_msg.rs
+++ b/components/msg/constellation_msg.rs
@@ -283,7 +283,7 @@ pub enum MozBrowserEvent {
     /// Sent when the SSL state changes within a browser `<iframe>`.
     SecurityChange,
     /// Sent when alert(), confirm(), or prompt() is called within a browser `<iframe>`.
-    ShowModalPrompt,
+    ShowModalPrompt(String, String, String, String), // TODO(simartin): Handle unblock()
     /// Sent when the document.title changes within a browser `<iframe>`.
     TitleChange(String),
     /// Sent when an HTTP authentification is requested.
@@ -305,7 +305,7 @@ impl MozBrowserEvent {
             MozBrowserEvent::LocationChange(_) => "mozbrowserlocationchange",
             MozBrowserEvent::OpenWindow => "mozbrowseropenwindow",
             MozBrowserEvent::SecurityChange => "mozbrowsersecuritychange",
-            MozBrowserEvent::ShowModalPrompt => "mozbrowsershowmodalprompt",
+            MozBrowserEvent::ShowModalPrompt(_, _, _, _) => "mozbrowsershowmodalprompt",
             MozBrowserEvent::TitleChange(_) => "mozbrowsertitlechange",
             MozBrowserEvent::UsernameAndPasswordRequired => "mozbrowserusernameandpasswordrequired",
             MozBrowserEvent::OpenSearch => "mozbrowseropensearch"
diff --git a/components/script/dom/htmliframeelement.rs b/components/script/dom/htmliframeelement.rs
index 34d668d7c80..172a6b1fbd6 100644
--- a/components/script/dom/htmliframeelement.rs
+++ b/components/script/dom/htmliframeelement.rs
@@ -4,6 +4,7 @@
 
 use dom::attr::{Attr, AttrValue};
 use dom::bindings::codegen::Bindings::BrowserElementBinding::BrowserElementIconChangeEventDetail;
+use dom::bindings::codegen::Bindings::BrowserElementBinding::BrowserShowModalPromptEventDetail;
 use dom::bindings::codegen::Bindings::HTMLIFrameElementBinding;
 use dom::bindings::codegen::Bindings::HTMLIFrameElementBinding::HTMLIFrameElementMethods;
 use dom::bindings::codegen::Bindings::WindowBinding::WindowMethods;
@@ -272,7 +273,7 @@ impl MozBrowserEventDetailBuilder for HTMLIFrameElement {
             MozBrowserEvent::AsyncScroll | MozBrowserEvent::Close | MozBrowserEvent::ContextMenu |
             MozBrowserEvent::Error | MozBrowserEvent::LoadEnd | MozBrowserEvent::LoadStart |
             MozBrowserEvent::OpenWindow | MozBrowserEvent::SecurityChange | MozBrowserEvent::OpenSearch  |
-            MozBrowserEvent::ShowModalPrompt | MozBrowserEvent::UsernameAndPasswordRequired => {
+            MozBrowserEvent::UsernameAndPasswordRequired => {
                 rval.set(NullValue());
             }
             MozBrowserEvent::LocationChange(ref string) | MozBrowserEvent::TitleChange(ref string) => {
@@ -285,6 +286,14 @@ impl MozBrowserEventDetailBuilder for HTMLIFrameElement {
                     sizes: Some(DOMString::from(sizes)),
                 }.to_jsval(cx, rval);
             }
+            MozBrowserEvent::ShowModalPrompt(prompt_type, title, message, return_value) => {
+                BrowserShowModalPromptEventDetail {
+                    promptType: Some(DOMString::from(prompt_type)),
+                    title: Some(DOMString::from(title)),
+                    message: Some(DOMString::from(message)),
+                    returnValue: Some(DOMString::from(return_value)),
+                }.to_jsval(cx, rval)
+            }
         }
     }
 }
diff --git a/components/script/dom/webidls/BrowserElement.webidl b/components/script/dom/webidls/BrowserElement.webidl
index 2ba6ceb1a01..ecd9df12e66 100644
--- a/components/script/dom/webidls/BrowserElement.webidl
+++ b/components/script/dom/webidls/BrowserElement.webidl
@@ -30,6 +30,14 @@ dictionary BrowserElementIconChangeEventDetail {
   DOMString sizes;
 };
 
+dictionary BrowserShowModalPromptEventDetail {
+  DOMString promptType;
+  DOMString title;
+  DOMString message;
+  DOMString returnValue;
+  // TODO(simartin) unblock() callback
+};
+
 BrowserElement implements BrowserElementCommon;
 BrowserElement implements BrowserElementPrivileged;
 
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index 36a7aafc080..602f815ebe7 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -45,7 +45,7 @@ use libc;
 use msg::ParseErrorReporter;
 use msg::compositor_msg::LayerId;
 use msg::constellation_msg::{ConstellationChan, DocumentState, LoadData};
-use msg::constellation_msg::{PipelineId, SubpageId, WindowSizeData};
+use msg::constellation_msg::{MozBrowserEvent, PipelineId, SubpageId, WindowSizeData};
 use msg::webdriver_msg::{WebDriverJSError, WebDriverJSResult};
 use net_traits::ResourceTask;
 use net_traits::image_cache_task::{ImageCacheChan, ImageCacheTask};
@@ -394,7 +394,7 @@ impl WindowMethods for Window {
     fn Alert(&self, s: DOMString) {
         // Right now, just print to the console
         // Ensure that stderr doesn't trample through the alert() we use to
-        // communicate test results.
+        // communicate test results (see executorservo.py in wptrunner).
         let stderr = stderr();
         let mut stderr = stderr.lock();
         let stdout = stdout();
@@ -402,6 +402,11 @@ impl WindowMethods for Window {
         writeln!(&mut stdout, "ALERT: {}", s).unwrap();
         stdout.flush().unwrap();
         stderr.flush().unwrap();
+
+        // https://developer.mozilla.org/en-US/docs/Web/Events/mozbrowsershowmodalprompt
+        let event = MozBrowserEvent::ShowModalPrompt("alert".to_owned(), "Alert".to_owned(),
+                                                     String::from(s), "".to_owned());
+        self.Document().trigger_mozbrowser_event(event);
     }
 
     // https://html.spec.whatwg.org/multipage/#dom-window-close
diff --git a/tests/wpt/mozilla/meta/MANIFEST.json b/tests/wpt/mozilla/meta/MANIFEST.json
index 64b128a17f9..5c9ded6ec90 100644
--- a/tests/wpt/mozilla/meta/MANIFEST.json
+++ b/tests/wpt/mozilla/meta/MANIFEST.json
@@ -5691,6 +5691,12 @@
             "url": "/_mozilla/mozilla/mozbrowser/mozbrowsericonchange_event.html"
           }
         ],
+        "mozilla/mozbrowser/mozbrowsershowmodalprompt_event.html": [
+          {
+            "path": "mozilla/mozbrowser/mozbrowsershowmodalprompt_event.html",
+            "url": "/_mozilla/mozilla/mozbrowser/mozbrowsershowmodalprompt_event.html"
+          }
+        ],
         "mozilla/mozbrowser/reload.html": [
           {
             "path": "mozilla/mozbrowser/reload.html",
diff --git a/tests/wpt/mozilla/tests/mozilla/mozbrowser/mozbrowsershowmodalprompt_event.html b/tests/wpt/mozilla/tests/mozilla/mozbrowser/mozbrowsershowmodalprompt_event.html
new file mode 100644
index 00000000000..731e508f876
--- /dev/null
+++ b/tests/wpt/mozilla/tests/mozilla/mozbrowser/mozbrowsershowmodalprompt_event.html
@@ -0,0 +1,19 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>Dispatch mozbrowsershowmodalprompt event for alert (issue #8352)</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<body></body>
+<script>
+    async_test(function(t) {
+        var iframe = document.createElement("iframe");
+        iframe.mozbrowser = "true";
+        iframe.src = "mozbrowsershowmodalprompt_event_iframe.html";
+        iframe.addEventListener("mozbrowsershowmodalprompt", t.step_func(e => {
+            assert_equals(e.detail.promptType, "alert");
+            assert_equals(e.detail.message, "my alert message");
+            t.done();
+        }));
+        document.body.appendChild(iframe);
+    });
+</script>
diff --git a/tests/wpt/mozilla/tests/mozilla/mozbrowser/mozbrowsershowmodalprompt_event_iframe.html b/tests/wpt/mozilla/tests/mozilla/mozbrowser/mozbrowsershowmodalprompt_event_iframe.html
new file mode 100644
index 00000000000..e9e8e84be54
--- /dev/null
+++ b/tests/wpt/mozilla/tests/mozilla/mozbrowser/mozbrowsershowmodalprompt_event_iframe.html
@@ -0,0 +1,6 @@
+<html>
+    <body></body>
+    <script>
+    window.alert("my alert message");
+    </script>
+</html>