diff -r 000000000000 -r 6474c204b198 browser/devtools/debugger/test/browser_dbg_panel-size.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/devtools/debugger/test/browser_dbg_panel-size.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,83 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Test that the sources and instruments panels widths are properly
+ * remembered when the debugger closes.
+ */
+
+const TAB_URL = EXAMPLE_URL + "doc_recursion-stack.html";
+
+function test() {
+  let gTab, gDebuggee, gPanel, gDebugger;
+  let gPrefs, gSources, gInstruments;
+
+  initDebugger(TAB_URL).then(([aTab, aDebuggee, aPanel]) => {
+    gTab = aTab;
+    gDebuggee = aDebuggee;
+    gPanel = aPanel;
+    gDebugger = gPanel.panelWin;
+    gPrefs = gDebugger.Prefs;
+    gSources = gDebugger.document.getElementById("sources-pane");
+    gInstruments = gDebugger.document.getElementById("instruments-pane");
+
+    waitForSourceShown(gPanel, ".html").then(performTest);
+  });
+
+  function performTest() {
+    let preferredSw = Services.prefs.getIntPref("devtools.debugger.ui.panes-sources-width");
+    let preferredIw = Services.prefs.getIntPref("devtools.debugger.ui.panes-instruments-width");
+    let someWidth1, someWidth2;
+
+    do {
+      someWidth1 = parseInt(Math.random() * 200) + 100;
+      someWidth2 = parseInt(Math.random() * 300) + 100;
+    } while ((someWidth1 == preferredSw) || (someWidth2 == preferredIw));
+
+    info("Preferred sources width: " + preferredSw);
+    info("Preferred instruments width: " + preferredIw);
+    info("Generated sources width: " + someWidth1);
+    info("Generated instruments width: " + someWidth2);
+
+    ok(gPrefs.sourcesWidth,
+      "The debugger preferences should have a saved sourcesWidth value.");
+    ok(gPrefs.instrumentsWidth,
+      "The debugger preferences should have a saved instrumentsWidth value.");
+
+    is(gPrefs.sourcesWidth, preferredSw,
+      "The debugger preferences should have a correct sourcesWidth value.");
+    is(gPrefs.instrumentsWidth, preferredIw,
+      "The debugger preferences should have a correct instrumentsWidth value.");
+
+    is(gSources.getAttribute("width"), gPrefs.sourcesWidth,
+      "The sources pane width should be the same as the preferred value.");
+    is(gInstruments.getAttribute("width"), gPrefs.instrumentsWidth,
+      "The instruments pane width should be the same as the preferred value.");
+
+    gSources.setAttribute("width", someWidth1);
+    gInstruments.setAttribute("width", someWidth2);
+
+    is(gPrefs.sourcesWidth, preferredSw,
+      "The sources pane width pref should still be the same as the preferred value.");
+    is(gPrefs.instrumentsWidth, preferredIw,
+      "The instruments pane width pref should still be the same as the preferred value.");
+
+    isnot(gSources.getAttribute("width"), gPrefs.sourcesWidth,
+      "The sources pane width should not be the preferred value anymore.");
+    isnot(gInstruments.getAttribute("width"), gPrefs.instrumentsWidth,
+      "The instruments pane width should not be the preferred value anymore.");
+
+    teardown(gPanel).then(() => {
+      is(gPrefs.sourcesWidth, someWidth1,
+        "The sources pane width should have been saved by now.");
+      is(gPrefs.instrumentsWidth, someWidth2,
+        "The instruments pane width should have been saved by now.");
+
+      // Cleanup after ourselves!
+      Services.prefs.setIntPref("devtools.debugger.ui.panes-sources-width", preferredSw);
+      Services.prefs.setIntPref("devtools.debugger.ui.panes-instruments-width", preferredIw);
+
+      finish();
+    });
+  }
+}