diff -r 000000000000 -r 6474c204b198 browser/devtools/debugger/test/browser_dbg_editor-mode.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/devtools/debugger/test/browser_dbg_editor-mode.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,93 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Make sure that updating the editor mode sets the right highlighting engine,
+ * and source URIs with extra query parameters also get the right engine.
+ */
+
+const TAB_URL = EXAMPLE_URL + "doc_editor-mode.html";
+
+let gTab, gDebuggee, gPanel, gDebugger;
+let gEditor, gSources;
+
+function test() {
+  initDebugger(TAB_URL).then(([aTab, aDebuggee, aPanel]) => {
+    gTab = aTab;
+    gDebuggee = aDebuggee;
+    gPanel = aPanel;
+    gDebugger = gPanel.panelWin;
+    gEditor = gDebugger.DebuggerView.editor;
+    gSources = gDebugger.DebuggerView.Sources;
+
+    waitForSourceAndCaretAndScopes(gPanel, "code_test-editor-mode", 1)
+      .then(testInitialSource)
+      .then(testSwitch1)
+      .then(testSwitch2)
+      .then(() => resumeDebuggerThenCloseAndFinish(gPanel))
+      .then(null, aError => {
+        ok(false, "Got an error: " + aError.message + "\n" + aError.stack);
+      });
+
+    gDebuggee.firstCall();
+  });
+}
+
+function testInitialSource() {
+  is(gSources.itemCount, 3,
+    "Found the expected number of sources.");
+
+  is(gEditor.getMode().name, "text",
+    "Found the expected editor mode.");
+  is(gEditor.getText().search(/firstCall/), -1,
+    "The first source is not displayed.");
+  is(gEditor.getText().search(/debugger/), 141,
+    "The second source is displayed.");
+  is(gEditor.getText().search(/banana/), -1,
+    "The third source is not displayed.");
+
+  let finished = waitForDebuggerEvents(gPanel, gDebugger.EVENTS.SOURCE_SHOWN);
+  gSources.selectedItem = e => e.attachment.label == "code_script-switching-01.js";
+  return finished;
+}
+
+function testSwitch1() {
+  is(gSources.itemCount, 3,
+    "Found the expected number of sources.");
+
+  is(gEditor.getMode().name, "javascript",
+    "Found the expected editor mode.");
+  is(gEditor.getText().search(/firstCall/), 118,
+    "The first source is displayed.");
+  is(gEditor.getText().search(/debugger/), -1,
+    "The second source is not displayed.");
+  is(gEditor.getText().search(/banana/), -1,
+    "The third source is not displayed.");
+
+  let finished = waitForDebuggerEvents(gPanel, gDebugger.EVENTS.SOURCE_SHOWN);
+  gSources.selectedItem = e => e.attachment.label == "doc_editor-mode.html";
+  return finished;
+}
+
+function testSwitch2() {
+  is(gSources.itemCount, 3,
+    "Found the expected number of sources.");
+
+  is(gEditor.getMode().name, "htmlmixed",
+    "Found the expected editor mode.");
+  is(gEditor.getText().search(/firstCall/), -1,
+    "The first source is not displayed.");
+  is(gEditor.getText().search(/debugger/), -1,
+    "The second source is not displayed.");
+  is(gEditor.getText().search(/banana/), 443,
+    "The third source is displayed.");
+}
+
+registerCleanupFunction(function() {
+  gTab = null;
+  gDebuggee = null;
+  gPanel = null;
+  gDebugger = null;
+  gEditor = null;
+  gSources = null;
+});