diff --git a/browser/base/content/test/general/head.js b/browser/base/content/test/general/head.js
index f6f407499cf3..a69409a47934 100644
--- a/browser/base/content/test/general/head.js
+++ b/browser/base/content/test/general/head.js
@@ -284,7 +284,7 @@ function is_hidden(element) {
   if (style.visibility != "visible") {
     return true;
   }
-  if (style.display == "-moz-popup") {
+  if (XULPopupElement.isInstance(element)) {
     return ["hiding", "closed"].includes(element.state);
   }
 
diff --git a/browser/components/customizableui/CustomizableUI.jsm b/browser/components/customizableui/CustomizableUI.jsm
index 9a2b2117a13d..e97c97936377 100644
--- a/browser/components/customizableui/CustomizableUI.jsm
+++ b/browser/components/customizableui/CustomizableUI.jsm
@@ -5596,7 +5596,7 @@ class OverflowableToolbar {
         let style = win.getComputedStyle(child);
         if (
           style.display == "none" ||
-          style.display == "-moz-popup" ||
+          win.XULPopupElement.isInstance(child) ||
           (style.position != "static" && style.position != "relative")
         ) {
           continue;
diff --git a/browser/components/customizableui/test/browser_PanelMultiView.js b/browser/components/customizableui/test/browser_PanelMultiView.js
index b3c1ccf1f581..645f09f03d9e 100644
--- a/browser/components/customizableui/test/browser_PanelMultiView.js
+++ b/browser/components/customizableui/test/browser_PanelMultiView.js
@@ -39,14 +39,15 @@ const EVENT_TYPES = [
  * they need to read style or layout information, like other code normally does.
  */
 function is_visible(element) {
-  var style = element.ownerGlobal.getComputedStyle(element);
+  let win = element.ownerGlobal;
+  let style = win.getComputedStyle(element);
   if (style.display == "none") {
     return false;
   }
   if (style.visibility != "visible") {
     return false;
   }
-  if (style.display == "-moz-popup" && element.state != "open") {
+  if (win.XULPopupElement.isInstance(element) && element.state != "open") {
     return false;
   }
 
diff --git a/browser/components/uitour/test/head.js b/browser/components/uitour/test/head.js
index 5157cf9ee4b9..9aa6d69e642b 100644
--- a/browser/components/uitour/test/head.js
+++ b/browser/components/uitour/test/head.js
@@ -68,14 +68,15 @@ function taskify(fun) {
 }
 
 function is_hidden(element) {
-  var style = element.ownerGlobal.getComputedStyle(element);
+  let win = element.ownerGlobal;
+  let style = win.getComputedStyle(element);
   if (style.display == "none") {
     return true;
   }
   if (style.visibility != "visible") {
     return true;
   }
-  if (style.display == "-moz-popup") {
+  if (win.XULPopupElement.isInstance(element)) {
     return ["hiding", "closed"].includes(element.state);
   }
 
@@ -88,14 +89,15 @@ function is_hidden(element) {
 }
 
 function is_visible(element) {
-  var style = element.ownerGlobal.getComputedStyle(element);
+  let win = element.ownerGlobal;
+  let style = win.getComputedStyle(element);
   if (style.display == "none") {
     return false;
   }
   if (style.visibility != "visible") {
     return false;
   }
-  if (style.display == "-moz-popup" && element.state != "open") {
+  if (win.XULPopupElement.isInstance(element) && element.state != "open") {
     return false;
   }
 
diff --git a/dom/events/EventStateManager.cpp b/dom/events/EventStateManager.cpp
index e2c8dfd4c37f..05fee4220a1c 100644
--- a/dom/events/EventStateManager.cpp
+++ b/dom/events/EventStateManager.cpp
@@ -3486,7 +3486,7 @@ nsresult EventStateManager::PostHandleEvent(nsPresContext* aPresContext,
 
           // If the mousedown happened inside a popup, don't try to set focus on
           // one of its containing elements
-          if (frame->StyleDisplay()->mDisplay == StyleDisplay::MozPopup) {
+          if (frame->IsMenuPopupFrame()) {
             newFocus = nullptr;
             break;
           }
diff --git a/layout/base/crashtests/306940-1.html b/layout/base/crashtests/306940-1.html
index f6197c37c4af..51cced408f71 100644
--- a/layout/base/crashtests/306940-1.html
+++ b/layout/base/crashtests/306940-1.html
@@ -8,14 +8,13 @@ function init()
   var c1 = document.getElementById("c1"); 
   var f1 = document.getElementById("f1"); 
   var a1 = document.getElementById("a1"); 
-  
+
   function first()
   {
-    f1.style.display = "-moz-popup";
     c1.style.height = "2em";
     window.status = "A";
   }
-  
+
   function second()
   {
     c1.style.position = "absolute";
diff --git a/layout/base/crashtests/372475-1.xhtml b/layout/base/crashtests/372475-1.xhtml
deleted file mode 100644
index ec3d7546313b..000000000000
--- a/layout/base/crashtests/372475-1.xhtml
+++ /dev/null
@@ -1,9 +0,0 @@
-<html xmlns="http://www.w3.org/1999/xhtml"
-      xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
-
-<body>
-
-<div style="display: -moz-popup"></div>
-
-</body>
-</html>
\ No newline at end of file
diff --git a/layout/base/crashtests/373628.html b/layout/base/crashtests/373628.html
index d71cc2518bdc..749af86b9ced 100644
--- a/layout/base/crashtests/373628.html
+++ b/layout/base/crashtests/373628.html
@@ -113,7 +113,7 @@ mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m
 </q>
 </q>
 </p>
-<p style=" display: -moz-popup;  direction: ltr;">
+<p style=" direction: ltr;">
 <q style=" display: table-header-group; ">
 <q style=" display: table-cell; ">
 <q style=" display: table; direction: ltr;">
@@ -314,7 +314,7 @@ mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m
 <s style=" position: fixed; direction: rtl;">
 <s style=" display: -moz-inline-box;  direction: rtl;">mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m 
 </s>
-<bdo style="overflow: hidden; display: -moz-popup; ">
+<bdo style="overflow: hidden;">
 </bdo>
 </s>
 </bdo>
@@ -345,7 +345,7 @@ mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m
 <bdo style=" display: -moz-box;">
 <bdo style="overflow: scroll; display: -moz-box; float: right;">
 <s style=" position: fixed; direction: rtl;">
-<bdo style="overflow: hidden; display: -moz-popup; ">
+<bdo style="overflow: hidden; ">
 <q style=" display: list-item; direction: ltr;">
 </q>
 </bdo>
@@ -378,12 +378,12 @@ mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m
 <bdo style=" display: -moz-box;">
 <bdo style="overflow: scroll; display: -moz-box; float: right;">
 <s style=" position: fixed; direction: rtl;">
-<bdo style="overflow: hidden; display: -moz-popup; ">
+<bdo style="overflow: hidden;">
 <q style=" display: list-item; direction: ltr;">mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m 
 </q>
 </bdo>
 </s>
-<bdo style="overflow: hidden; display: -moz-popup; ">
+<bdo style="overflow: hidden;">
 <q style=" display: list-item; direction: ltr;">mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m 
 </q>
 </bdo>
@@ -414,7 +414,7 @@ mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m
 <s style=" display: table-cell;">
 <bdo style=" display: -moz-box;">
 <bdo style="overflow: scroll; display: -moz-box; float: right;">
-<bdo style="overflow: hidden; display: -moz-popup; ">
+<bdo style="overflow: hidden;">
 <q style=" display: list-item; direction: ltr;">mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m 
 </q>
 </bdo>
@@ -448,7 +448,7 @@ mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m
 <s style=" display: table-cell;">
 <bdo style=" display: -moz-box;">
 <bdo style="overflow: scroll; display: -moz-box; float: right;">
-<bdo style="overflow: hidden; display: -moz-popup; ">
+<bdo style="overflow: hidden;">
 <q style=" display: list-item; direction: ltr;">mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m 
 </q>
 </bdo>
@@ -479,7 +479,7 @@ mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m
 <s style=" display: table-cell;">
 <bdo style=" display: -moz-box;">
 <bdo style="overflow: scroll; display: -moz-box; float: right;">
-<bdo style="overflow: hidden; display: -moz-popup; ">
+<bdo style="overflow: hidden;">
 <q style=" display: list-item; direction: ltr;">mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m 
 <samp style=" display: inherit; position: fixed; direction: ltr;">mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m 
 <nobr style=" display: -moz-inline-box; position: absolute; direction: ltr;">
@@ -553,7 +553,7 @@ mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m
 <s style=" display: table-cell;">
 <bdo style=" display: -moz-box;">
 <bdo style="overflow: scroll; display: -moz-box; float: right;">
-<bdo style="overflow: hidden; display: -moz-popup; ">
+<bdo style="overflow: hidden;">
 <q style=" display: list-item; direction: ltr;">
 <s style="overflow: auto; display: inline-table; ">
 <s style=" display: table-column-group; direction: rtl;">
@@ -598,7 +598,7 @@ mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m
 <s style=" display: table-cell;">
 <bdo style=" display: -moz-box;">
 <bdo style="overflow: scroll; display: -moz-box; float: right;">
-<bdo style="overflow: hidden; display: -moz-popup; ">
+<bdo style="overflow: hidden;">
 <q style=" display: list-item; direction: ltr;">
 <s style="overflow: auto; display: inline-table; ">
 <s style=" display: table-column-group; direction: rtl;">
@@ -639,7 +639,7 @@ mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m
 <s style=" display: table-cell;">
 <bdo style=" display: -moz-box;">
 <bdo style="overflow: scroll; display: -moz-box; float: right;">
-<bdo style="overflow: hidden; display: -moz-popup; ">
+<bdo style="overflow: hidden;">
 <q style=" display: list-item; direction: ltr;">
 <s style="overflow: auto; display: inline-table; ">
 <s style=" display: table-column-group; direction: rtl;">
@@ -699,7 +699,7 @@ mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m
 <s style=" display: table-cell;">
 <bdo style=" display: -moz-box;">
 <bdo style="overflow: scroll; display: -moz-box; float: right;">
-<bdo style="overflow: hidden; display: -moz-popup; ">
+<bdo style="overflow: hidden;">
 <q style=" display: list-item; direction: ltr;">
 <s style="overflow: auto; display: inline-table; ">
 <s style=" display: table-column-group; direction: rtl;">
@@ -740,7 +740,7 @@ mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m
 <s style=" display: table-cell;">
 <bdo style=" display: -moz-box;">
 <bdo style="overflow: scroll; display: -moz-box; float: right;">
-<bdo style="overflow: hidden; display: -moz-popup; ">
+<bdo style="overflow: hidden;">
 <q style=" display: list-item; direction: ltr;">
 <s style="overflow: auto; display: inline-table; ">
 <s style=" display: table-column-group; direction: rtl;">
@@ -780,7 +780,7 @@ mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m
 <s style=" display: table-cell;">
 <bdo style=" display: -moz-box;">
 <bdo style="overflow: scroll; display: -moz-box; float: right;">
-<bdo style="overflow: hidden; display: -moz-popup; ">
+<bdo style="overflow: hidden;">
 <q style=" display: list-item; direction: ltr;">
 <s style="overflow: auto; display: inline-table; ">
 <s style=" display: table-column-group; direction: rtl;">
@@ -880,7 +880,7 @@ mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m
 <s style=" display: table-cell;">
 <bdo style=" display: -moz-box;">
 <bdo style="overflow: scroll; display: -moz-box; float: right;">
-<bdo style="overflow: hidden; display: -moz-popup; ">
+<bdo style="overflow: hidden;">
 <s style="overflow: auto; display: inline-table; ">
 <s style=" display: table-column-group; direction: rtl;">
 <bdo style=" display: inline-block; direction: rtl;">
@@ -919,7 +919,7 @@ mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m mm m
 <s style=" display: table-cell;">
 <bdo style=" display: -moz-box;">
 <bdo style="overflow: scroll; display: -moz-box; float: right;">
-<bdo style="overflow: hidden; display: -moz-popup; ">
+<bdo style="overflow: hidden;">
 <s style="overflow: auto; display: inline-table; ">
 <s style=" display: table-column-group; direction: rtl;">
 <bdo style=" display: inline-block; direction: rtl;">
diff --git a/layout/base/crashtests/405049-1.xhtml b/layout/base/crashtests/405049-1.xhtml
index fb68de11eb9b..07d2f4b18d5f 100644
--- a/layout/base/crashtests/405049-1.xhtml
+++ b/layout/base/crashtests/405049-1.xhtml
@@ -1,3 +1,3 @@
 <window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" style="display: table;">
-<box style="display: -moz-popup;"/>
-</window>
\ No newline at end of file
+  <panel/>
+</window>
diff --git a/layout/base/crashtests/613817-1.svg b/layout/base/crashtests/613817-1.svg
deleted file mode 100644
index 9f5165ffe5fa..000000000000
--- a/layout/base/crashtests/613817-1.svg
+++ /dev/null
@@ -1,12 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg"><foreignObject id="fo">
-  <div xmlns="http://www.w3.org/1999/xhtml" style="display: -moz-popup;"></div></foreignObject><script>
-
-function boom()
-{
-  document.getElementById("fo").style.MozAppearance = "menuitem";
-}
-
-window.addEventListener("load", boom, false);
-
-</script></svg>
-
diff --git a/layout/base/crashtests/crashtests.list b/layout/base/crashtests/crashtests.list
index 9218e01ea33c..cfd122e75d78 100644
--- a/layout/base/crashtests/crashtests.list
+++ b/layout/base/crashtests/crashtests.list
@@ -137,7 +137,6 @@ load 369547-2.html
 load 369945-1.xhtml
 load 371681-1.xhtml
 load 372237-1.html
-load 372475-1.xhtml
 load 372550-1.html
 load 373628-1.html
 load 374297-1.html
@@ -328,7 +327,6 @@ load 595039-1.html
 load 597924-1.html
 load 606432-1.html
 load 609821-1.xhtml
-load 613817-1.svg
 load 615146-1.html
 load 615781-1.xhtml
 load 616495-single-side-composite-color-border.html
diff --git a/layout/base/nsCSSFrameConstructor.cpp b/layout/base/nsCSSFrameConstructor.cpp
index 5559bd199e79..ec13c68c8511 100644
--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -2399,14 +2399,12 @@ nsIFrame* nsCSSFrameConstructor::ConstructDocElementFrame(
   } else if (display->mDisplay == StyleDisplay::Flex ||
              display->mDisplay == StyleDisplay::WebkitBox ||
              display->mDisplay == StyleDisplay::Grid ||
-             display->mDisplay == StyleDisplay::MozBox ||
-             display->mDisplay == StyleDisplay::MozPopup) {
+             display->mDisplay == StyleDisplay::MozBox) {
     auto func = [&] {
       if (display->mDisplay == StyleDisplay::Grid) {
         return NS_NewGridContainerFrame;
       }
-      if ((display->mDisplay == StyleDisplay::MozBox ||
-           display->mDisplay == StyleDisplay::MozPopup) &&
+      if (display->mDisplay == StyleDisplay::MozBox &&
           !computedStyle->StyleVisibility()->EmulateMozBoxWithFlex()) {
         return NS_NewBoxFrame;
       }
@@ -4022,6 +4020,8 @@ const nsCSSFrameConstructor::FrameConstructionData*
 nsCSSFrameConstructor::FindXULTagData(const Element& aElement,
                                       ComputedStyle& aStyle) {
   MOZ_ASSERT(aElement.IsXULElement());
+  static constexpr FrameConstructionData kPopupData(
+      NS_NewMenuPopupFrame, FCDATA_IS_POPUP | FCDATA_SKIP_ABSPOS_PUSH);
 
   static constexpr FrameConstructionDataByTag sXULTagData[] = {
       SIMPLE_XUL_CREATE(image, NS_NewImageBoxFrame),
@@ -4047,7 +4047,11 @@ nsCSSFrameConstructor::FindXULTagData(const Element& aElement,
       SIMPLE_XUL_CREATE(splitter, NS_NewSplitterFrame),
       SIMPLE_XUL_CREATE(slider, NS_NewSliderFrame),
       SIMPLE_XUL_CREATE(scrollbar, NS_NewScrollbarFrame),
-      SIMPLE_XUL_CREATE(scrollbarbutton, NS_NewScrollbarButtonFrame)};
+      SIMPLE_XUL_CREATE(scrollbarbutton, NS_NewScrollbarButtonFrame),
+      {nsGkAtoms::panel, kPopupData},
+      {nsGkAtoms::menupopup, kPopupData},
+      {nsGkAtoms::tooltip, kPopupData},
+  };
 
   return FindDataByTag(aElement, aStyle, sXULTagData, ArrayLength(sXULTagData));
 }
@@ -4457,11 +4461,6 @@ nsCSSFrameConstructor::FindDisplayData(const nsStyleDisplay& aDisplay,
           FCDATA_DESIRED_PARENT_TYPE_TO_BITS(eTypeRuby));
       return &data;
     }
-    case StyleDisplayInside::MozPopup: {
-      static constexpr FrameConstructionData data(
-          NS_NewMenuPopupFrame, FCDATA_IS_POPUP | FCDATA_SKIP_ABSPOS_PUSH);
-      return &data;
-    }
     default:
       MOZ_ASSERT_UNREACHABLE("unknown 'display' value");
       return nullptr;
@@ -5465,15 +5464,6 @@ void nsCSSFrameConstructor::ConstructFramesFromItem(
     nsContainerFrame* aParentFrame, nsFrameList& aFrameList) {
   FrameConstructionItem& item = aIter.item();
   ComputedStyle* computedStyle = item.mComputedStyle;
-
-  const auto* disp = computedStyle->StyleDisplay();
-  MOZ_ASSERT(!disp->IsAbsolutelyPositionedStyle() ||
-                 disp->DisplayInside() != StyleDisplayInside::MozBox,
-             "This may be a frame that was previously blockified "
-             "but isn't any longer! It probably needs explicit "
-             "'display:block' to preserve behavior");
-  Unused << disp;  // (unused in configs that define the assertion away)
-
   if (item.mIsText) {
     // If this is collapsible whitespace next to a line boundary,
     // don't create a frame. item.IsWhitespace() also sets the
@@ -5843,10 +5833,6 @@ bool nsCSSFrameConstructor::IsValidSibling(nsIFrame* aSibling,
     }
 
     StyleDisplay display = aDisplay.value();
-    if (LayoutFrameType::Menu == parentType) {
-      return (StyleDisplay::MozPopup == display) ==
-             (StyleDisplay::MozPopup == siblingDisplay);
-    }
     // To have decent performance we want to return false in cases in which
     // reordering the two siblings has no effect on display.  To ensure
     // correctness, we MUST return false in cases where the two siblings have
diff --git a/layout/base/nsFrameTraversal.cpp b/layout/base/nsFrameTraversal.cpp
index c770feb845bd..f380e61684da 100644
--- a/layout/base/nsFrameTraversal.cpp
+++ b/layout/base/nsFrameTraversal.cpp
@@ -422,8 +422,7 @@ bool nsFrameIterator::IsPopupFrame(nsIFrame* aFrame) {
   if (mSkipPopupChecks) {
     return false;
   }
-
-  return (aFrame && aFrame->StyleDisplay()->mDisplay == StyleDisplay::MozPopup);
+  return aFrame && aFrame->IsMenuPopupFrame();
 }
 
 // nsVisualIterator implementation
diff --git a/layout/reftests/bugs/490173-1-ref.html b/layout/reftests/bugs/490173-1-ref.html
deleted file mode 100644
index 73bace2f3add..000000000000
--- a/layout/reftests/bugs/490173-1-ref.html
+++ /dev/null
@@ -1,8 +0,0 @@
-<!DOCTYPE html>
-<html>
-<head>
-</head>
-<body>
-<div>Foo</div>
-</body>
-</html>
diff --git a/layout/reftests/bugs/490173-1.html b/layout/reftests/bugs/490173-1.html
deleted file mode 100644
index 4bef4d61218b..000000000000
--- a/layout/reftests/bugs/490173-1.html
+++ /dev/null
@@ -1,8 +0,0 @@
-<!DOCTYPE html>
-<html>
-<head>
-</head>
-<body onload="document.getElementById('f1').style.display = '';">
-<div id="f1" style="display: -moz-popup;">Foo</div>
-</body>
-</html>
diff --git a/layout/reftests/bugs/reftest.list b/layout/reftests/bugs/reftest.list
index 24d3f9c86d81..18e0a115f9a4 100644
--- a/layout/reftests/bugs/reftest.list
+++ b/layout/reftests/bugs/reftest.list
@@ -1314,7 +1314,6 @@ fuzzy(0-1,0-5) == 482659-1d.html 482659-1-ref.html
 == 488685-1.html 488685-1-ref.html
 == 488692-1.html 488692-1-ref.html
 == 489868-1.svg 489868-1-ref.svg
-== 490173-1.html 490173-1-ref.html
 == 490173-2.html 490173-2-ref.html
 == 490176-1.html 490176-1-ref.html
 == 490177-1.svg 490177-1-ref.svg
diff --git a/layout/reftests/table-anonymous-boxes/162063-1.xhtml b/layout/reftests/table-anonymous-boxes/162063-1.xhtml
index 6a83f3a6f51b..387ad4e6ec33 100644
--- a/layout/reftests/table-anonymous-boxes/162063-1.xhtml
+++ b/layout/reftests/table-anonymous-boxes/162063-1.xhtml
@@ -68,7 +68,7 @@ window.addEventListener("MozReftestInvalidate", boom);
  </div>
  
  <div style="display:table; border-spacing:10px; background-color:teal">
- <div id="target10" style="display:-moz-popup">target</div>
+ <div id="target10">target</div>
  </div>
  
  <div style="display:table; border-spacing:10px; background-color:SkyBlue ">
diff --git a/layout/style/nsStyleConsts.h b/layout/style/nsStyleConsts.h
index d0b963b7710f..07e89619bed6 100644
--- a/layout/style/nsStyleConsts.h
+++ b/layout/style/nsStyleConsts.h
@@ -91,8 +91,6 @@ enum class StyleDisplay : uint16_t {
       StyleDisplayFrom(StyleDisplayOutside::Block, StyleDisplayInside::MozBox),
   MozInlineBox =
       StyleDisplayFrom(StyleDisplayOutside::Inline, StyleDisplayInside::MozBox),
-  MozPopup =
-      StyleDisplayFrom(StyleDisplayOutside::XUL, StyleDisplayInside::MozPopup),
 };
 // The order of the StyleDisplay values isn't meaningful.
 bool operator<(const StyleDisplay&, const StyleDisplay&) = delete;
diff --git a/layout/style/test/test_non_content_accessible_values.html b/layout/style/test/test_non_content_accessible_values.html
index c1f640be730c..4307d5852693 100644
--- a/layout/style/test/test_non_content_accessible_values.html
+++ b/layout/style/test/test_non_content_accessible_values.html
@@ -19,7 +19,6 @@ const NON_CONTENT_ACCESSIBLE_VALUES = {
     "-moz-autofill-background",
   ],
   "display": [
-    "-moz-popup",
     "-moz-box",
     "-moz-inline-box",
   ],
diff --git a/layout/xul/crashtests/350460.xhtml b/layout/xul/crashtests/350460.xhtml
deleted file mode 100644
index b13de6c977fd..000000000000
--- a/layout/xul/crashtests/350460.xhtml
+++ /dev/null
@@ -1,8 +0,0 @@
-<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
- title="Crash [@ DoDeletingFrameSubtree] after reloading a xul page a few times with display: -moz-popup and menuitem">
-  <menuitem style="display: -moz-popup;">
-    <box style="display: -moz-popup;">
-      <box style="display: -moz-popup;"/>
-    </box>
-  </menuitem>
-</window>
\ No newline at end of file
diff --git a/layout/xul/crashtests/crashtests.list b/layout/xul/crashtests/crashtests.list
index 04797e55040f..975b88fc4024 100644
--- a/layout/xul/crashtests/crashtests.list
+++ b/layout/xul/crashtests/crashtests.list
@@ -16,7 +16,6 @@ skip-if(Android) load chrome://reftest/content/crashtests/layout/xul/crashtests/
 load 329407-1.xml
 load chrome://reftest/content/crashtests/layout/xul/crashtests/336962-1.xhtml
 skip-if(Android) load chrome://reftest/content/crashtests/layout/xul/crashtests/344228-1.xhtml
-load chrome://reftest/content/crashtests/layout/xul/crashtests/350460.xhtml
 skip-if(Android) load chrome://reftest/content/crashtests/layout/xul/crashtests/365151.xhtml
 load chrome://reftest/content/crashtests/layout/xul/crashtests/366112-1.xhtml
 skip-if(Android) load chrome://reftest/content/crashtests/layout/xul/crashtests/366203-1.xhtml
diff --git a/layout/xul/test/chrome.ini b/layout/xul/test/chrome.ini
index 5732f2507204..b0ebb99b0711 100644
--- a/layout/xul/test/chrome.ini
+++ b/layout/xul/test/chrome.ini
@@ -15,7 +15,6 @@ support-files =
 
 [test_blockify_moz_box.html]
 [test_bug159346.xhtml]
-[test_bug372685.xhtml]
 [test_bug381167.xhtml]
 [test_bug398982-1.xhtml]
 [test_bug398982-2.xhtml]
diff --git a/layout/xul/test/test_bug372685.xhtml b/layout/xul/test/test_bug372685.xhtml
deleted file mode 100644
index 56d4d8434660..000000000000
--- a/layout/xul/test/test_bug372685.xhtml
+++ /dev/null
@@ -1,49 +0,0 @@
-<?xml version="1.0"?>
-<?xml-stylesheet href="chrome://global/skin" type="text/css"?>
-<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css" type="text/css"?>
-<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
-        xmlns:html="http://www.w3.org/1999/xhtml"
-        title="Test for Bug 372685">
-<!--
-https://bugzilla.mozilla.org/show_bug.cgi?id=372685
--->
-
-  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
-
-<menuitem id="a" style="display: -moz-box;">
-<box id="b" style="display: -moz-popup; ">
-<box id="c" style="position: fixed; display: block;"></box>
-</box>
-</menuitem>
-
-<script class="testbody" type="application/javascript">
-<![CDATA[
-
-function removestyles(i){
-          document.getElementById('a').removeAttribute('style');
-	  var x=document.getElementById('html_body').offsetHeight;
-	  is(0, 0, "this is a crash test, so always ok if we survive this far");
-          SimpleTest.finish();
-}
-function do_test() {
-          setTimeout(removestyles,200);
-}
-
-SimpleTest.waitForExplicitFinish();
-
-]]>
-</script>
-
-<body  id="html_body" xmlns="http://www.w3.org/1999/xhtml">
-<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=372685">Mozilla Bug 372685</a>
-<p id="display"></p>
-
-<pre id="test">
-</pre>
-<script>
-addLoadEvent(do_test);
-</script>
-</body>
-
-
-</window>
diff --git a/servo/components/style/values/specified/box.rs b/servo/components/style/values/specified/box.rs
index 285c4a84a199..d275c311dc29 100644
--- a/servo/components/style/values/specified/box.rs
+++ b/servo/components/style/values/specified/box.rs
@@ -104,8 +104,6 @@ pub enum DisplayInside {
     WebkitBox,
     #[cfg(feature = "gecko")]
     MozBox,
-    #[cfg(feature = "gecko")]
-    MozPopup,
 }
 
 #[allow(missing_docs)]
@@ -217,8 +215,6 @@ impl Display {
     pub const MozBox: Self = Self::new(DisplayOutside::Block, DisplayInside::MozBox);
     #[cfg(feature = "gecko")]
     pub const MozInlineBox: Self = Self::new(DisplayOutside::Inline, DisplayInside::MozBox);
-    #[cfg(feature = "gecko")]
-    pub const MozPopup: Self = Self::new(DisplayOutside::XUL, DisplayInside::MozPopup);
 
     /// Make a raw display value from <display-outside> and <display-inside> values.
     #[inline]
@@ -597,8 +593,6 @@ impl Parse for Display {
             "-moz-box" if moz_display_values_enabled(context) => Display::MozBox,
             #[cfg(feature = "gecko")]
             "-moz-inline-box" if moz_display_values_enabled(context) => Display::MozInlineBox,
-            #[cfg(feature = "gecko")]
-            "-moz-popup" if moz_display_values_enabled(context) => Display::MozPopup,
         })
     }
 }
diff --git a/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm b/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm
index 7bb997820b94..6bb8b4e3ca78 100644
--- a/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm
+++ b/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm
@@ -295,14 +295,15 @@ var BrowserTestUtils = {
    * @return {boolean}
    */
   is_hidden(element) {
-    var style = element.ownerGlobal.getComputedStyle(element);
+    let win = element.ownerGlobal;
+    let style = win.getComputedStyle(element);
     if (style.display == "none") {
       return true;
     }
     if (style.visibility != "visible") {
       return true;
     }
-    if (style.display == "-moz-popup") {
+    if (win.XULPopupElement.isInstance(element)) {
       return ["hiding", "closed"].includes(element.state);
     }
 
@@ -323,14 +324,15 @@ var BrowserTestUtils = {
    * @return {boolean}
    */
   is_visible(element) {
-    var style = element.ownerGlobal.getComputedStyle(element);
+    let win = element.ownerGlobal;
+    let style = win.getComputedStyle(element);
     if (style.display == "none") {
       return false;
     }
     if (style.visibility != "visible") {
       return false;
     }
-    if (style.display == "-moz-popup" && element.state != "open") {
+    if (win.XULPopupElement.isInstance(element) && element.state != "open") {
       return false;
     }
 
diff --git a/toolkit/components/narrate/test/NarrateTestUtils.jsm b/toolkit/components/narrate/test/NarrateTestUtils.jsm
index 228b44546b70..8b42f0a15946 100644
--- a/toolkit/components/narrate/test/NarrateTestUtils.jsm
+++ b/toolkit/components/narrate/test/NarrateTestUtils.jsm
@@ -27,12 +27,15 @@ var NarrateTestUtils = {
   FORWARD: ".narrate-skip-next",
 
   isVisible(element) {
-    let style = element.ownerGlobal.getComputedStyle(element);
+    let win = element.ownerGlobal;
+    let style = win.getComputedStyle(element);
     if (style.display == "none") {
       return false;
-    } else if (style.visibility != "visible") {
+    }
+    if (style.visibility != "visible") {
       return false;
-    } else if (style.display == "-moz-popup" && element.state != "open") {
+    }
+    if (win.XULPopupElement.isInstance(element) && element.state != "open") {
       return false;
     }
 
diff --git a/toolkit/content/widgets/button.js b/toolkit/content/widgets/button.js
index 2f1c8369112c..da29495d0644 100644
--- a/toolkit/content/widgets/button.js
+++ b/toolkit/content/widgets/button.js
@@ -182,7 +182,7 @@
         return NodeFilter.FILTER_REJECT;
       }
       // but it may be a popup element, in which case we look at "state"...
-      if (cs.display == "-moz-popup" && node.state != "open") {
+      if (XULPopupElement.isInstance(node) && node.state != "open") {
         return NodeFilter.FILTER_REJECT;
       }
       // OK - the node seems visible, so it is a candidate.
diff --git a/toolkit/content/xul.css b/toolkit/content/xul.css
index 8d96457c6648..63d95bb59c2c 100644
--- a/toolkit/content/xul.css
+++ b/toolkit/content/xul.css
@@ -270,7 +270,6 @@ panel {
 menupopup,
 panel,
 tooltip {
-  display: -moz-popup;
   position: absolute;
   -moz-top-layer: top;
   /* Popups can't have overflow */