--- a/layout/forms/nsComboboxControlFrame.cpp
+++ b/layout/forms/nsComboboxControlFrame.cpp
@@ -43,16 +43,17 @@
 #include "nsThemeConstants.h"
 #include "mozilla/Likely.h"
 #include <algorithm>
 #include "nsTextNode.h"
 #include "mozilla/AsyncEventDispatcher.h"
 #include "mozilla/EventStates.h"
 #include "mozilla/LookAndFeel.h"
 #include "mozilla/MouseEvents.h"
+#include "mozilla/Preferences.h"
 #include "mozilla/Unused.h"
 #include "gfx2DGlue.h"
 #include "mozilla/widget/nsAutoRollup.h"
 
 #ifdef XP_WIN
 #define COMBOBOX_ROLLUP_CONSUME_EVENT 0
 #else
 #define COMBOBOX_ROLLUP_CONSUME_EVENT 1
@@ -235,16 +236,23 @@ nsComboboxControlFrame::nsComboboxContro
   , mDisplayedIndex(-1)
   , mLastDropDownBeforeScreenBCoord(nscoord_MIN)
   , mLastDropDownAfterScreenBCoord(nscoord_MIN)
   , mDroppedDown(false)
   , mInRedisplayText(false)
   , mDelayedShowDropDown(false)
   , mIsOpenInParentProcess(false)
 {
+#ifdef DEBUG
+  if (nsLayoutUtils::IsContentSelectEnabled()) {
+    mAlwaysDroppedDown = Preferences::GetBool("layout.always_show_select_popup.enabled");
+    mDroppedDown |= mAlwaysDroppedDown;
+  }
+#endif
+
   REFLOW_COUNTER_INIT()
 }
 
 //--------------------------------------------------------------
 nsComboboxControlFrame::~nsComboboxControlFrame()
 {
   REFLOW_COUNTER_DUMP("nsCCF");
 }
@@ -340,16 +348,19 @@ nsComboboxControlFrame::ShowList(bool aS
 
   // TODO(kuoe0) Remove this function when content-select is enabled.
   //
   // This function is used to handle the widget/view stuff, so we just return
   // when content-select is enabled. And the following callee, ShowPopup(), will
   // also be ignored, it is only used to show and hide the widget.
   if (nsLayoutUtils::IsContentSelectEnabled()) {
     mDroppedDown = aShowList;
+#ifdef DEBUG
+    mDroppedDown |= mAlwaysDroppedDown;
+#endif
     return true;
   }
 
   nsView* view = mDropdownFrame->GetView();
   if (aShowList) {
     NS_ASSERTION(!view->HasWidget(),
                  "We shouldn't have a widget before we need to display the popup");
 

--- a/layout/forms/nsComboboxControlFrame.h
+++ b/layout/forms/nsComboboxControlFrame.h
@@ -315,16 +315,21 @@ protected:
   bool                  mDroppedDown;
   // See comment in HandleRedisplayTextEvent().
   bool                  mInRedisplayText;
   // Acting on ShowDropDown(true) is delayed until we're focused.
   bool                  mDelayedShowDropDown;
 
   bool                  mIsOpenInParentProcess;
 
+#ifdef DEBUG
+  // Always show the dropdown frame
+  bool                  mAlwaysDroppedDown;
+#endif
+
   // static class data member for Bug 32920
   // only one control can be focused at a time
   static nsComboboxControlFrame* sFocused;
 
 #ifdef DO_REFLOW_COUNTER
   int32_t mReflowId;
 #endif
 };

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -1334,16 +1334,21 @@ pref("dom.forms.selectSearch", false);
 pref("dom.forms.select.customstyling", false);
 #else
 pref("dom.forms.select.customstyling", true);
 #endif
 pref("dom.select_popup_in_parent.enabled", false);
 // Bug 1421229 - content-select
 pref("dom.select_popup_in_content.enabled", false);
 
+#ifdef DEBUG
+// Always show the select popup
+pref("layout.always_show_select_popup.enabled", false);
+#endif
+
 // Enable Directory API. By default, disabled.
 pref("dom.input.dirpicker", false);
 
 // Enable not moving the cursor to end when a text input or textarea has .value
 // set to the value it already has.  By default, enabled.
 pref("dom.input.skip_cursor_move_for_same_value_set", true);
 
 pref("dom.cycle_collector.incremental", true);