diff --git a/browser/devtools/netmonitor/netmonitor-view.js b/browser/devtools/netmonitor/netmonitor-view.js
index 30d46a71a068..34a0d94a018b 100644
--- a/browser/devtools/netmonitor/netmonitor-view.js
+++ b/browser/devtools/netmonitor/netmonitor-view.js
@@ -1315,6 +1315,7 @@ NetworkDetailsView.prototype = {
     $("#request-params-box").setAttribute("flex", "1");
     $("#request-params-box").hidden = false;
     $("#request-post-data-textarea-box").hidden = true;
+    $("#response-content-info-header").hidden = true;
     $("#response-content-json-box").hidden = true;
     $("#response-content-textarea-box").hidden = true;
     $("#response-content-image-box").hidden = true;
@@ -1612,18 +1613,42 @@ NetworkDetailsView.prototype = {
     gNetwork.getString(text).then((aString) => {
       // Handle json.
       if (mimeType.contains("/json")) {
-        $("#response-content-json-box").hidden = false;
         let jsonpRegex = /^[a-zA-Z0-9_$]+\(|\)$/g; // JSONP with callback.
         let sanitizedJSON = aString.replace(jsonpRegex, "");
         let callbackPadding = aString.match(jsonpRegex);
 
-        let jsonScopeName = callbackPadding
-          ? L10N.getFormatStr("jsonpScopeName", callbackPadding[0].slice(0, -1))
-          : L10N.getStr("jsonScopeName");
+        // Make sure this is an valid JSON object first. If so, nicely display
+        // the parsing results in a variables view. Otherwise, simply show
+        // the contents as plain text.
+        try {
+          var jsonObject = JSON.parse(sanitizedJSON);
+        } catch (e) {
+          var parsingError = e;
+        }
 
-        let jsonScope = this._json.addScope(jsonScopeName);
-        jsonScope.addVar().populate(JSON.parse(sanitizedJSON), { expanded: true });
-        jsonScope.expanded = true;
+        // Valid JSON.
+        if (jsonObject) {
+          $("#response-content-json-box").hidden = false;
+          let jsonScopeName = callbackPadding
+            ? L10N.getFormatStr("jsonpScopeName", callbackPadding[0].slice(0, -1))
+            : L10N.getStr("jsonScopeName");
+
+          let jsonScope = this._json.addScope(jsonScopeName);
+          jsonScope.addVar().populate(jsonObject, { expanded: true });
+          jsonScope.expanded = true;
+        }
+        // Malformed JSON.
+        else {
+          $("#response-content-textarea-box").hidden = false;
+          NetMonitorView.editor("#response-content-textarea").then((aEditor) => {
+            aEditor.setMode(SourceEditor.MODES.JAVASCRIPT);
+            aEditor.setText(aString);
+          });
+          let infoHeader = $("#response-content-info-header");
+          infoHeader.setAttribute("value", parsingError);
+          infoHeader.setAttribute("tooltiptext", parsingError);
+          infoHeader.hidden = false;
+        }
       }
       // Handle images.
       else if (mimeType.contains("image/")) {
diff --git a/browser/devtools/netmonitor/netmonitor.xul b/browser/devtools/netmonitor/netmonitor.xul
index 6749ef3d613c..1349b429a174 100644
--- a/browser/devtools/netmonitor/netmonitor.xul
+++ b/browser/devtools/netmonitor/netmonitor.xul
@@ -272,6 +272,7 @@
         <tabpanel id="response-tabpanel"
                   class="tabpanel-content">
           <vbox flex="1">
+            <label id="response-content-info-header"/>
             <vbox id="response-content-json-box" flex="1" hidden="true">
               <vbox id="response-content-json" flex="1"/>
             </vbox>
diff --git a/browser/devtools/netmonitor/test/Makefile.in b/browser/devtools/netmonitor/test/Makefile.in
index 1347aa0a7f68..0fcd2e895ad4 100644
--- a/browser/devtools/netmonitor/test/Makefile.in
+++ b/browser/devtools/netmonitor/test/Makefile.in
@@ -31,6 +31,7 @@ MOCHITEST_BROWSER_TESTS = \
 	browser_net_post-data-02.js \
 	browser_net_jsonp.js \
 	browser_net_json-long.js \
+	browser_net_json-malformed.js \
 	browser_net_timeline_ticks.js \
 	browser_net_sort-01.js \
 	browser_net_sort-02.js \
@@ -55,6 +56,7 @@ MOCHITEST_BROWSER_PAGES = \
 	html_post-raw-test-page.html \
 	html_jsonp-test-page.html \
 	html_json-long-test-page.html \
+	html_json-malformed-test-page.html \
 	html_sorting-test-page.html \
 	html_filter-test-page.html \
 	html_infinite-get-page.html \
diff --git a/browser/devtools/netmonitor/test/browser_net_content-type.js b/browser/devtools/netmonitor/test/browser_net_content-type.js
index a506193a2bca..3c7ed2d6216d 100644
--- a/browser/devtools/netmonitor/test/browser_net_content-type.js
+++ b/browser/devtools/netmonitor/test/browser_net_content-type.js
@@ -109,14 +109,15 @@ function test() {
           "The response tab in the network details pane should be selected.");
 
         function checkVisibility(aBox) {
+          is(tabpanel.querySelector("#response-content-info-header")
+            .hasAttribute("hidden"), true,
+            "The response info header doesn't have the intended visibility.");
           is(tabpanel.querySelector("#response-content-json-box")
             .hasAttribute("hidden"), aBox != "json",
             "The response content json box doesn't have the intended visibility.");
-
           is(tabpanel.querySelector("#response-content-textarea-box")
             .hasAttribute("hidden"), aBox != "textarea",
             "The response content textarea box doesn't have the intended visibility.");
-
           is(tabpanel.querySelector("#response-content-image-box")
             .hasAttribute("hidden"), aBox != "image",
             "The response content image box doesn't have the intended visibility.");
diff --git a/browser/devtools/netmonitor/test/browser_net_json-long.js b/browser/devtools/netmonitor/test/browser_net_json-long.js
index 63caa81a81c2..c683b8cdac86 100644
--- a/browser/devtools/netmonitor/test/browser_net_json-long.js
+++ b/browser/devtools/netmonitor/test/browser_net_json-long.js
@@ -46,14 +46,15 @@ function test() {
         is(tab.getAttribute("selected"), "true",
           "The response tab in the network details pane should be selected.");
 
+        is(tabpanel.querySelector("#response-content-info-header")
+          .hasAttribute("hidden"), true,
+          "The response info header doesn't have the intended visibility.");
         is(tabpanel.querySelector("#response-content-json-box")
           .hasAttribute("hidden"), false,
           "The response content json box doesn't have the intended visibility.");
-
         is(tabpanel.querySelector("#response-content-textarea-box")
           .hasAttribute("hidden"), true,
           "The response content textarea box doesn't have the intended visibility.");
-
         is(tabpanel.querySelector("#response-content-image-box")
           .hasAttribute("hidden"), true,
           "The response content image box doesn't have the intended visibility.");
diff --git a/browser/devtools/netmonitor/test/browser_net_json-malformed.js b/browser/devtools/netmonitor/test/browser_net_json-malformed.js
new file mode 100644
index 000000000000..1ea3124d4f8f
--- /dev/null
+++ b/browser/devtools/netmonitor/test/browser_net_json-malformed.js
@@ -0,0 +1,71 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Tests if malformed JSON responses are handled correctly.
+ */
+
+function test() {
+  initNetMonitor(JSON_MALFORMED_URL).then(([aTab, aDebuggee, aMonitor]) => {
+    info("Starting test... ");
+
+    let { document, SourceEditor, NetMonitorView } = aMonitor.panelWin;
+    let { RequestsMenu } = NetMonitorView;
+
+    RequestsMenu.lazyUpdate = false;
+
+    waitForNetworkEvents(aMonitor, 1).then(() => {
+      verifyRequestItemTarget(RequestsMenu.getItemAtIndex(0),
+        "GET", CONTENT_TYPE_SJS + "?fmt=json-malformed", {
+          status: 200,
+          statusText: "OK",
+          type: "json",
+          fullMimeType: "text/json; charset=utf-8"
+        });
+
+      EventUtils.sendMouseEvent({ type: "mousedown" },
+        document.getElementById("details-pane-toggle"));
+      EventUtils.sendMouseEvent({ type: "mousedown" },
+        document.querySelectorAll("#details-pane tab")[3]);
+
+      let tab = document.querySelectorAll("#details-pane tab")[3];
+      let tabpanel = document.querySelectorAll("#details-pane tabpanel")[3];
+
+      is(tab.getAttribute("selected"), "true",
+        "The response tab in the network details pane should be selected.");
+
+      is(tabpanel.querySelector("#response-content-info-header")
+        .hasAttribute("hidden"), false,
+        "The response info header doesn't have the intended visibility.");
+      is(tabpanel.querySelector("#response-content-info-header")
+        .getAttribute("value"),
+        "SyntaxError: JSON.parse: unexpected non-whitespace character after JSON data",
+        "The response info header doesn't have the intended value attribute.");
+      is(tabpanel.querySelector("#response-content-info-header")
+        .getAttribute("tooltiptext"),
+        "SyntaxError: JSON.parse: unexpected non-whitespace character after JSON data",
+        "The response info header doesn't have the intended tooltiptext attribute.");
+
+      is(tabpanel.querySelector("#response-content-json-box")
+        .hasAttribute("hidden"), true,
+        "The response content json box doesn't have the intended visibility.");
+      is(tabpanel.querySelector("#response-content-textarea-box")
+        .hasAttribute("hidden"), false,
+        "The response content textarea box doesn't have the intended visibility.");
+      is(tabpanel.querySelector("#response-content-image-box")
+        .hasAttribute("hidden"), true,
+        "The response content image box doesn't have the intended visibility.");
+
+      NetMonitorView.editor("#response-content-textarea").then((aEditor) => {
+        is(aEditor.getText(), "{ \"greeting\": \"Hello malformed JSON!\" },",
+          "The text shown in the source editor is incorrect.");
+        is(aEditor.getMode(), SourceEditor.MODES.JAVASCRIPT,
+          "The mode active in the source editor is incorrect.");
+
+        teardown(aMonitor).then(finish);
+      });
+    });
+
+    aDebuggee.performRequests();
+  });
+}
diff --git a/browser/devtools/netmonitor/test/browser_net_jsonp.js b/browser/devtools/netmonitor/test/browser_net_jsonp.js
index 3f578668e7bc..a3b3f03c9f96 100644
--- a/browser/devtools/netmonitor/test/browser_net_jsonp.js
+++ b/browser/devtools/netmonitor/test/browser_net_jsonp.js
@@ -40,14 +40,15 @@ function test() {
         is(tab.getAttribute("selected"), "true",
           "The response tab in the network details pane should be selected.");
 
+        is(tabpanel.querySelector("#response-content-info-header")
+          .hasAttribute("hidden"), true,
+          "The response info header doesn't have the intended visibility.");
         is(tabpanel.querySelector("#response-content-json-box")
           .hasAttribute("hidden"), false,
           "The response content json box doesn't have the intended visibility.");
-
         is(tabpanel.querySelector("#response-content-textarea-box")
           .hasAttribute("hidden"), true,
           "The response content textarea box doesn't have the intended visibility.");
-
         is(tabpanel.querySelector("#response-content-image-box")
           .hasAttribute("hidden"), true,
           "The response content image box doesn't have the intended visibility.");
diff --git a/browser/devtools/netmonitor/test/browser_net_post-data-01.js b/browser/devtools/netmonitor/test/browser_net_post-data-01.js
index 585534f249eb..1e23adeb3552 100644
--- a/browser/devtools/netmonitor/test/browser_net_post-data-01.js
+++ b/browser/devtools/netmonitor/test/browser_net_post-data-01.js
@@ -61,7 +61,6 @@ function test() {
           is(tabpanel.querySelector("#request-params-box")
             .hasAttribute("hidden"), !aBox.contains("params"),
             "The request params box doesn't have the indended visibility.");
-
           is(tabpanel.querySelector("#request-post-data-textarea-box")
             .hasAttribute("hidden"), !aBox.contains("textarea"),
             "The request post data textarea box doesn't have the indended visibility.");
diff --git a/browser/devtools/netmonitor/test/browser_net_simple-request-details.js b/browser/devtools/netmonitor/test/browser_net_simple-request-details.js
index d8f41f41fed8..c43afb22258d 100644
--- a/browser/devtools/netmonitor/test/browser_net_simple-request-details.js
+++ b/browser/devtools/netmonitor/test/browser_net_simple-request-details.js
@@ -163,7 +163,6 @@ function test() {
       is(tabpanel.querySelector("#request-params-box")
         .hasAttribute("hidden"), false,
         "The request params box should not be hidden.");
-
       is(tabpanel.querySelector("#request-post-data-textarea-box")
         .hasAttribute("hidden"), true,
         "The request post data textarea box should be hidden.");
@@ -179,14 +178,15 @@ function test() {
       is(tab.getAttribute("selected"), "true",
         "The response tab in the network details pane should be selected.");
 
+      is(tabpanel.querySelector("#response-content-info-header")
+        .hasAttribute("hidden"), true,
+        "The response info header should be hidden.");
       is(tabpanel.querySelector("#response-content-json-box")
         .hasAttribute("hidden"), true,
         "The response content json box should be hidden.");
-
       is(tabpanel.querySelector("#response-content-textarea-box")
         .hasAttribute("hidden"), false,
         "The response content textarea box should not be hidden.");
-
       is(tabpanel.querySelector("#response-content-image-box")
         .hasAttribute("hidden"), true,
         "The response content image box should be hidden.");
diff --git a/browser/devtools/netmonitor/test/browser_net_status-codes.js b/browser/devtools/netmonitor/test/browser_net_status-codes.js
index 5f01f2343115..0e62f80718ad 100644
--- a/browser/devtools/netmonitor/test/browser_net_status-codes.js
+++ b/browser/devtools/netmonitor/test/browser_net_status-codes.js
@@ -144,7 +144,6 @@ function test() {
         is(tabpanel.querySelector("#request-params-box")
           .hasAttribute("hidden"), false,
           "The request params box should not be hidden.");
-
         is(tabpanel.querySelector("#request-post-data-textarea-box")
           .hasAttribute("hidden"), true,
           "The request post data textarea box should be hidden.");
diff --git a/browser/devtools/netmonitor/test/head.js b/browser/devtools/netmonitor/test/head.js
index d3ca5f1f5ed6..ce8fcd5fc141 100644
--- a/browser/devtools/netmonitor/test/head.js
+++ b/browser/devtools/netmonitor/test/head.js
@@ -22,6 +22,7 @@ const POST_DATA_URL = EXAMPLE_URL + "html_post-data-test-page.html";
 const POST_RAW_URL = EXAMPLE_URL + "html_post-raw-test-page.html";
 const JSONP_URL = EXAMPLE_URL + "html_jsonp-test-page.html";
 const JSON_LONG_URL = EXAMPLE_URL + "html_json-long-test-page.html";
+const JSON_MALFORMED_URL = EXAMPLE_URL + "html_json-malformed-test-page.html";
 const SORTING_URL = EXAMPLE_URL + "html_sorting-test-page.html";
 const FILTERING_URL = EXAMPLE_URL + "html_filter-test-page.html";
 const INFINITE_GET_URL = EXAMPLE_URL + "html_infinite-get-page.html";
diff --git a/browser/devtools/netmonitor/test/html_json-malformed-test-page.html b/browser/devtools/netmonitor/test/html_json-malformed-test-page.html
new file mode 100644
index 000000000000..3c47c797fea1
--- /dev/null
+++ b/browser/devtools/netmonitor/test/html_json-malformed-test-page.html
@@ -0,0 +1,33 @@
+<!doctype html>
+
+<html>
+  <head>
+    <meta charset="utf-8"/>
+    <title>Network Monitor test page</title>
+  </head>
+
+  <body>
+    <p>JSON malformed test</p>
+
+    <script type="text/javascript">
+      function get(aAddress, aCallback) {
+        var xhr = new XMLHttpRequest();
+        xhr.open("GET", aAddress, true);
+
+        xhr.onreadystatechange = function() {
+          if (this.readyState == this.DONE) {
+            aCallback();
+          }
+        };
+        xhr.send(null);
+      }
+
+      function performRequests() {
+        get("sjs_content-type-test-server.sjs?fmt=json-malformed", function() {
+          // Done.
+        });
+      }
+    </script>
+  </body>
+
+</html>
diff --git a/browser/devtools/netmonitor/test/sjs_content-type-test-server.sjs b/browser/devtools/netmonitor/test/sjs_content-type-test-server.sjs
index 01274136b4d8..a0cb4ec65607 100644
--- a/browser/devtools/netmonitor/test/sjs_content-type-test-server.sjs
+++ b/browser/devtools/netmonitor/test/sjs_content-type-test-server.sjs
@@ -78,6 +78,13 @@ function handleRequest(request, response) {
         response.finish();
         break;
       }
+      case "json-malformed": {
+        response.setStatusLine(request.httpVersion, 200, "OK");
+        response.setHeader("Content-Type", "text/json; charset=utf-8", false);
+        response.write("{ \"greeting\": \"Hello malformed JSON!\" },");
+        response.finish();
+        break;
+      }
       case "font": {
         response.setStatusLine(request.httpVersion, 200, "OK");
         response.setHeader("Content-Type", "font/woff", false);
diff --git a/browser/themes/linux/devtools/netmonitor.css b/browser/themes/linux/devtools/netmonitor.css
index bb17c33b3288..b7c2e9698ad5 100644
--- a/browser/themes/linux/devtools/netmonitor.css
+++ b/browser/themes/linux/devtools/netmonitor.css
@@ -381,6 +381,17 @@ box.requests-menu-status[code^="5"] {
 
 /* Response tabpanel */
 
+#response-content-info-header {
+  background:
+    url(background-noise-toolbar.png),
+    linear-gradient(hsl(0,61%,40%), hsl(0,61%,31%)) repeat-x top left;
+  box-shadow:
+    inset 0 1px 0 hsla(210,40%,83%,.15),
+    inset 0 -1px 0 hsla(210,40%,83%,.05);
+  margin: 0;
+  padding: 5px 8px;
+}
+
 #response-content-image-box {
   padding-top: 10px;
   padding-bottom: 10px;
diff --git a/browser/themes/osx/devtools/netmonitor.css b/browser/themes/osx/devtools/netmonitor.css
index 96d2f713c93b..e77a7bc68879 100644
--- a/browser/themes/osx/devtools/netmonitor.css
+++ b/browser/themes/osx/devtools/netmonitor.css
@@ -381,6 +381,17 @@ box.requests-menu-status[code^="5"] {
 
 /* Response tabpanel */
 
+#response-content-info-header {
+  background:
+    url(background-noise-toolbar.png),
+    linear-gradient(hsl(0,61%,40%), hsl(0,61%,31%)) repeat-x top left;
+  box-shadow:
+    inset 0 1px 0 hsla(210,40%,83%,.15),
+    inset 0 -1px 0 hsla(210,40%,83%,.05);
+  margin: 0;
+  padding: 5px 8px;
+}
+
 #response-content-image-box {
   padding-top: 10px;
   padding-bottom: 10px;
diff --git a/browser/themes/windows/devtools/netmonitor.css b/browser/themes/windows/devtools/netmonitor.css
index 04ae79b92397..411e3fdf30eb 100644
--- a/browser/themes/windows/devtools/netmonitor.css
+++ b/browser/themes/windows/devtools/netmonitor.css
@@ -381,6 +381,17 @@ box.requests-menu-status[code^="5"] {
 
 /* Response tabpanel */
 
+#response-content-info-header {
+  background:
+    url(background-noise-toolbar.png),
+    linear-gradient(hsl(0,61%,40%), hsl(0,61%,31%)) repeat-x top left;
+  box-shadow:
+    inset 0 1px 0 hsla(210,40%,83%,.15),
+    inset 0 -1px 0 hsla(210,40%,83%,.05);
+  margin: 0;
+  padding: 5px 8px;
+}
+
 #response-content-image-box {
   padding-top: 10px;
   padding-bottom: 10px;