diff --git a/browser/devtools/styleeditor/StyleEditorDebuggee.jsm b/browser/devtools/styleeditor/StyleEditorDebuggee.jsm
index ff7a9be6035f..272f3322b175 100644
--- a/browser/devtools/styleeditor/StyleEditorDebuggee.jsm
+++ b/browser/devtools/styleeditor/StyleEditorDebuggee.jsm
@@ -95,12 +95,17 @@ StyleEditorDebuggee.prototype = {
    */
   clear: function() {
     this.baseURI = null;
+    this.clearStyleSheets();
+  },
 
+  /**
+   * Clear stylesheets.
+   */
+  clearStyleSheets: function() {
     for (let stylesheet of this.styleSheets) {
       stylesheet.destroy();
     }
     this.styleSheets = [];
-
     this.emit("stylesheets-cleared");
   },
 
@@ -137,6 +142,9 @@ StyleEditorDebuggee.prototype = {
    *         Object with 'styleSheets' array of actor forms
    */
   _onDocumentLoad: function(type, request) {
+    if (this.styleSheets.length > 0) {
+      this.clearStyleSheets();
+    }
     let sheets = [];
     for (let form of request.styleSheets) {
       let sheet = this._addStyleSheet(form);
diff --git a/browser/devtools/styleeditor/test/Makefile.in b/browser/devtools/styleeditor/test/Makefile.in
index 519ce166c171..1918c406a217 100644
--- a/browser/devtools/styleeditor/test/Makefile.in
+++ b/browser/devtools/styleeditor/test/Makefile.in
@@ -26,6 +26,7 @@ MOCHITEST_BROWSER_FILES := \
                  browser_styleeditor_sv_resize.js \
                  browser_styleeditor_bug_740541_iframes.js \
                  browser_styleeditor_bug_851132_middle_click.js \
+                 browser_styleeditor_bug_870339.js \
                  browser_styleeditor_nostyle.js \
                  browser_styleeditor_reload.js \
                  head.js \
diff --git a/browser/devtools/styleeditor/test/browser_styleeditor_bug_870339.js b/browser/devtools/styleeditor/test/browser_styleeditor_bug_870339.js
new file mode 100644
index 000000000000..6daa0f4f8b99
--- /dev/null
+++ b/browser/devtools/styleeditor/test/browser_styleeditor_bug_870339.js
@@ -0,0 +1,46 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+function test()
+{
+  const SIMPLE = TEST_BASE_HTTP + "simple.css";
+  const DOCUMENT_WITH_ONE_STYLESHEET = "data:text/html;charset=UTF-8," +
+          encodeURIComponent(
+            ["<!DOCTYPE html>",
+             "<html>",
+             " <head>",
+             "  <title>Bug 870339</title>",
+             '  <link rel="stylesheet" type="text/css" href="'+SIMPLE+'">',
+             " </head>",
+             " <body>",
+             " </body>",
+             "</html>"
+            ].join("\n"));
+
+  waitForExplicitFinish();
+  addTabAndOpenStyleEditor(function (aPanel) {
+    let debuggee = aPanel._debuggee;
+
+    // Spam the _onNewDocument callback multiple times before the
+    // StyleEditorActor has a chance to respond to the first one.
+    const SPAM_COUNT = 2;
+    for (let i=0; i<SPAM_COUNT; ++i) {
+      debuggee._onNewDocument();
+    }
+
+    // Wait for the StyleEditorActor to respond to each "newDocument"
+    // message.
+    let loadCount = 0;
+    debuggee.on("document-load", function () {
+      ++loadCount;
+      if (loadCount == SPAM_COUNT) {
+        // No matter how large SPAM_COUNT is, the number of style
+        // sheets should never be more than the number of style sheets
+        // in the document.
+        is(debuggee.styleSheets.length, 1, "correct style sheet count");
+        finish();
+      }
+    });
+  });
+  content.location = DOCUMENT_WITH_ONE_STYLESHEET;
+}
