--- a/dom/base/nsDeprecatedOperationList.h
+++ b/dom/base/nsDeprecatedOperationList.h
@@ -48,9 +48,10 @@ DEPRECATED_OPERATION(MozRequestFullScree
 DEPRECATED_OPERATION(MozfullscreenchangeDeprecatedPrefix)
 DEPRECATED_OPERATION(MozfullscreenerrorDeprecatedPrefix)
 DEPRECATED_OPERATION(External_AddSearchProvider)
 DEPRECATED_OPERATION(MouseEvent_MozPressure)
 DEPRECATED_OPERATION(MathML_DeprecatedAlignmentAttributes)
 DEPRECATED_OPERATION(MathML_DeprecatedLineThicknessValue)
 DEPRECATED_OPERATION(MathML_DeprecatedMathSizeValue)
 DEPRECATED_OPERATION(MathML_DeprecatedMathSpaceValue)
+DEPRECATED_OPERATION(MathML_DeprecatedMencloseNotationRadical)
 DEPRECATED_OPERATION(MathML_DeprecatedStyleAttribute)

--- a/dom/locales/en-US/chrome/dom/dom.properties
+++ b/dom/locales/en-US/chrome/dom/dom.properties
@@ -373,10 +373,12 @@ MouseEvent_MozPressureWarning=MouseEvent
 MathML_DeprecatedAlignmentAttributesWarning=MathML attributes “align”, “numalign” and “denomalign” are deprecated values and will be removed at a future date.
 # LOCALIZATION NOTE: Do not translate thin, medium, thick and linethickness.
 MathML_DeprecatedLineThicknessValueWarning=“thin”, “medium” and “thick” are deprecated values for the linethickness attribute and will be removed at a future date.
 # LOCALIZATION NOTE: Do not translate small, normal, big and mathsize.
 MathML_DeprecatedMathSizeValueWarning=“small”, “normal” and “big” are deprecated values for the mathsize attribute and will be removed at a future date.
 # LOCALIZATION NOTE: Do not translate veryverythinmathspace, verythinmathspace,
 # thinmathspace, mediummathspace, thickmathspace, verythickmathspace, veryverythickmathspace and MathML.
 MathML_DeprecatedMathSpaceValueWarning=“veryverythinmathspace”, “verythinmathspace”, “thinmathspace”, “mediummathspace”, “thickmathspace”, “verythickmathspace” and “veryverythickmathspace” are deprecated values for MathML lengths and will be removed at a future date.
+# LOCALIZATION NOTE: Do not translate radical, notation and menclose.
+MathML_DeprecatedMencloseNotationRadical=The “radical” value is deprecated for the “notation” attribute of the <menclose> element and will be removed at a future date.
 # LOCALIZATION NOTE: Do not translate MathML, background, color, fontfamily, fontsize, fontstyle and fontweight.
 MathML_DeprecatedStyleAttributeWarning=MathML attributes “background”, “color”, “fontfamily”, “fontsize”, “fontstyle” and “fontweight” are deprecated and will be removed at a future date.

--- a/layout/mathml/nsMathMLmencloseFrame.cpp
+++ b/layout/mathml/nsMathMLmencloseFrame.cpp
@@ -4,16 +4,17 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "nsMathMLmencloseFrame.h"
 
 #include "gfx2DGlue.h"
 #include "gfxUtils.h"
 #include "mozilla/PresShell.h"
+#include "mozilla/StaticPrefs_mathml.h"
 #include "mozilla/gfx/2D.h"
 #include "mozilla/gfx/PathHelpers.h"
 #include "nsPresContext.h"
 #include "nsWhitespaceTokenizer.h"
 
 #include "nsDisplayList.h"
 #include "gfxContext.h"
 #include "nsMathMLChar.h"
@@ -100,19 +101,22 @@ nsresult nsMathMLmencloseFrame::AddNotat
   if (aNotation.EqualsLiteral("longdiv")) {
     rv = AllocateMathMLChar(NOTATION_LONGDIV);
     NS_ENSURE_SUCCESS(rv, rv);
     mNotationsToDraw += NOTATION_LONGDIV;
   } else if (aNotation.EqualsLiteral("actuarial")) {
     mNotationsToDraw += NOTATION_RIGHT;
     mNotationsToDraw += NOTATION_TOP;
   } else if (aNotation.EqualsLiteral("radical")) {
-    rv = AllocateMathMLChar(NOTATION_RADICAL);
-    NS_ENSURE_SUCCESS(rv, rv);
-    mNotationsToDraw += NOTATION_RADICAL;
+    if (!StaticPrefs::mathml_deprecated_menclose_notation_radical_disabled()) {
+      mContent->OwnerDoc()->WarnOnceAbout(dom::Document::eMathML_DeprecatedMencloseNotationRadical);
+      rv = AllocateMathMLChar(NOTATION_RADICAL);
+      NS_ENSURE_SUCCESS(rv, rv);
+      mNotationsToDraw += NOTATION_RADICAL;
+    }
   } else if (aNotation.EqualsLiteral("box")) {
     mNotationsToDraw += NOTATION_LEFT;
     mNotationsToDraw += NOTATION_RIGHT;
     mNotationsToDraw += NOTATION_TOP;
     mNotationsToDraw += NOTATION_BOTTOM;
   } else if (aNotation.EqualsLiteral("roundedbox")) {
     mNotationsToDraw += NOTATION_ROUNDEDBOX;
   } else if (aNotation.EqualsLiteral("circle")) {

--- a/layout/mathml/nsMathMLmsqrtFrame.h
+++ b/layout/mathml/nsMathMLmsqrtFrame.h
@@ -32,16 +32,19 @@ 2.3.4).
 
 The <mroot> element increments scriptlevel by 2, and sets displaystyle to
 "false", within index, but leaves both attributes unchanged within base. The
 <msqrt> element leaves both attributes unchanged within all its arguments.
 These attributes are inherited by every element from its rendering environment,
 but can be set explicitly only on <mstyle>. (See Section 3.3.4.)
 */
 
+// XXXfredw: This class should share its layout logic with nsMathMLmrootFrame
+// when the menclose "radical" notation is removed.
+// See https://bugzilla.mozilla.org/show_bug.cgi?id=1548522
 class nsMathMLmsqrtFrame final : public nsMathMLmencloseFrame {
  public:
   NS_DECL_FRAMEARENA_HELPERS(nsMathMLmsqrtFrame)
 
   friend nsIFrame* NS_NewMathMLmsqrtFrame(mozilla::PresShell* aPresShell,
                                           ComputedStyle* aStyle);
 
   virtual void Init(nsIContent* aContent, nsContainerFrame* aParent,

--- a/layout/reftests/mathml/reftest.list
+++ b/layout/reftests/mathml/reftest.list
@@ -12,17 +12,17 @@ random-if(/^Windows\x20NT\x206\.1/.test(
 == 373533-2.xhtml about:blank
 == 373533-3.xhtml about:blank
 random-if(/^Windows\x20NT\x206\.1/.test(http.oscpu)) == 393760-1.xml 393760-1-ref.xml # Bug 1392106
 fuzzy-if(skiaContent,0-2,0-529) == 393760-2.xml 393760-2-ref.xml
 == 414123.xhtml 414123-ref.xhtml
 == dir-1.html dir-1-ref.html
 pref(mathml.deprecated_alignment_attributes.disabled,false) random-if(/^Windows\x20NT\x206\.1/.test(http.oscpu)) == dir-2.html dir-2-ref.html # Bug 1392106
 random-if(gtkWidget) == dir-3.html dir-3-ref.html # bug 1309426
-== dir-4.html dir-4-ref.html
+pref(mathml.deprecated_menclose_notation_radical.disabled,false) == dir-4.html dir-4-ref.html
 == dir-5.html dir-5-ref.html
 fuzzy-if(cocoaWidget,0-135,0-56) == dir-6a.html dir-6a-ref.html
 == css-spacing-1.html css-spacing-1-ref.html
 pref(mathml.disabled,true) == disabled-scriptlevel-1.html disabled-scriptlevel-1-ref.html
 pref(mathml.disabled,true) == disabled-scriptlevel-1.xhtml disabled-scriptlevel-1-ref.xhtml
 random-if(smallScreen&&Android) fuzzy(0-255,0-200) fuzzy-if(geckoview&&webrender,201-216,312-316) fuzzy-if(webrender&&winWidget,114-255,245-361) fuzzy-if(webrender&&OSX,79-153,307-314) == mirror-op-1.html mirror-op-1-ref.html
 != mirror-op-2.html mirror-op-2-ref.html
 != mirror-op-3.html mirror-op-3-ref.html
@@ -202,17 +202,17 @@ random-if(winWidget&&!d2d) == opentype-s
 == operator-1.xhtml operator-1-ref.xhtml
 == scriptshift-1.xhtml scriptshift-1-ref.xhtml
 pref(mathml.nonzero_unitless_lengths.disabled,false) == number-size-1.xhtml number-size-1-ref.xhtml
 fuzzy-if(skiaContent,0-1,0-80) fuzzy-if(Android,0-255,0-105) skip-if(winWidget) == multiscripts-1.html multiscripts-1-ref.html # Windows: bug 1314684; Android: bug 1392254
 == mathml-mmultiscript-base.html mathml-mmultiscript-base-ref.html
 == mathml-mmultiscript-mprescript.html mathml-mmultiscript-mprescript-ref.html
 != menclose-1a.html menclose-1-ref.html
 != menclose-1b.html menclose-1-ref.html
-!= menclose-1c.html menclose-1-ref.html
+pref(mathml.deprecated_menclose_notation_radical.disabled,false) != menclose-1c.html menclose-1-ref.html
 != menclose-1d.html menclose-1-ref.html
 != menclose-1e.html menclose-1-ref.html
 != menclose-1f.html menclose-1-ref.html
 != menclose-1g.html menclose-1-ref.html
 != menclose-1h.html menclose-1-ref.html
 != menclose-1i.html menclose-1-ref.html
 != menclose-1j.html menclose-1-ref.html
 != menclose-1k.html menclose-1-ref.html
@@ -235,49 +235,49 @@ random-if(gtkWidget) == menclose-2-round
 == menclose-2-top.html menclose-2-top-ref.html
 random-if(gtkWidget) == menclose-2-updiagonalarrow.html menclose-2-updiagonalarrow-ref.html # bug 1309426
 == menclose-2-updiagonalstrike.html menclose-2-updiagonalstrike-ref.html
 == menclose-2-verticalstrike.html menclose-2-verticalstrike-ref.html
 random-if(gtkWidget) == menclose-2-roundedbox.html menclose-2-roundedbox-ref.html # bug 1309426
 == menclose-2-phasorangle.html menclose-2-phasorangle-ref.html
 == menclose-3-box.html menclose-3-box-ref.html
 == menclose-3-madruwb.html menclose-3-madruwb-ref.html
-fails-if(Android) == menclose-3-radical.html menclose-3-radical-ref.html # Bug 973917
+pref(mathml.deprecated_menclose_notation_radical.disabled,false) fails-if(Android) == menclose-3-radical.html menclose-3-radical-ref.html # Bug 973917
 == menclose-3-default.html menclose-3-default-ref.html
 == menclose-3-invalid.html menclose-3-invalid-ref.html
 == menclose-3-multiple.html menclose-3-multiple-ref.html
 == menclose-3-unknown.html menclose-3-unknown.html
 == menclose-4.html data:text/html,<body>Pass
 == menclose-5-actuarial.html menclose-5-actuarial-ref.html
 == menclose-5-bottom.html menclose-5-bottom-ref.html
 == menclose-5-box.html menclose-5-box-ref.html
 fuzzy-if(OSX,0-1,0-100) fuzzy-if(skiaContent,0-1,0-2) == menclose-5-circle.html menclose-5-circle-ref.html
 fuzzy-if(d2d,0-7,0-1) == menclose-5-downdiagonalstrike.html menclose-5-downdiagonalstrike-ref.html
 == menclose-5-horizontalstrike.html menclose-5-horizontalstrike-ref.html
 == menclose-5-left.html menclose-5-left-ref.html
 == menclose-5-longdiv.html menclose-5-longdiv-ref.html
 == menclose-5-madruwb.html menclose-5-madruwb-ref.html
-== menclose-5-radical.html menclose-5-radical-ref.html
+pref(mathml.deprecated_menclose_notation_radical.disabled,false) == menclose-5-radical.html menclose-5-radical-ref.html
 == menclose-5-right.html menclose-5-right-ref.html
 fuzzy-if(skiaContent,0-5,0-5) == menclose-5-roundedbox.html menclose-5-roundedbox-ref.html
 == menclose-5-top.html menclose-5-top-ref.html
 fuzzy-if(skiaContent,0-1,0-1) == menclose-5-updiagonalarrow.html menclose-5-updiagonalarrow-ref.html
 fuzzy-if(d2d,0-7,0-1) == menclose-5-updiagonalstrike.html menclose-5-updiagonalstrike-ref.html
 == menclose-5-verticalstrike.html menclose-5-verticalstrike-ref.html
 == menclose-5-phasorangle.html menclose-5-phasorangle-ref.html
 == menclose-6-actuarial.html menclose-6-ref.html
 == menclose-6-bottom.html menclose-6-ref.html
 == menclose-6-box.html menclose-6-ref.html
 fuzzy-if(skiaContent,0-1,0-2) == menclose-6-circle.html menclose-6-ref.html
 fuzzy-if(d2d,0-7,0-1) == menclose-6-downdiagonalstrike.html menclose-6-ref.html
 == menclose-6-horizontalstrike.html menclose-6-ref.html
 == menclose-6-left.html menclose-6-ref.html
 == menclose-6-longdiv.html menclose-6-ref.html
 == menclose-6-madruwb.html menclose-6-ref.html
-== menclose-6-radical.html menclose-6-ref.html
+pref(mathml.deprecated_menclose_notation_radical.disabled,false) == menclose-6-radical.html menclose-6-ref.html
 == menclose-6-right.html menclose-6-ref.html
 fuzzy-if(skiaContent,0-2,0-3) == menclose-6-roundedbox.html menclose-6-ref.html
 == menclose-6-top.html menclose-6-ref.html
 == menclose-6-updiagonalarrow.html menclose-6-ref.html
 fuzzy-if(d2d,0-7,0-1) == menclose-6-updiagonalstrike.html menclose-6-ref.html
 == menclose-6-verticalstrike.html menclose-6-ref.html
 == menclose-6-phasorangle.html menclose-6-ref.html
 == mmultiscript-align.html mmultiscript-align-ref.html

--- a/modules/libpref/init/StaticPrefList.yaml
+++ b/modules/libpref/init/StaticPrefList.yaml
@@ -5296,16 +5296,22 @@
 
 # Whether to disable deprecated style attributes background, color, fontfamily,
 # fontsize, fontstyle and fontweight.
 - name: mathml.deprecated_style_attributes.disabled
   type: bool
   value: @IS_NIGHTLY_BUILD@
   mirror: always
 
+# Whether to disable deprecated "radical" notation for the menclose element.
+- name: mathml.deprecated_menclose_notation_radical.disabled
+  type: bool
+  value: @IS_NIGHTLY_BUILD@
+  mirror: always
+
 # Whether to disable legacy MathML number values that are not valid CSS numbers
 # (e.g. "1234.")
 - name: mathml.legacy_number_syntax.disabled
   type: bool
   value: true
   mirror: always
 
 # Whether to disable legacy names "small", "normal" and "big" for the

--- a/testing/web-platform/meta/mathml/__dir__.ini
+++ b/testing/web-platform/meta/mathml/__dir__.ini
@@ -1,1 +1,1 @@
-prefs: [mathml.deprecated_style_attributes.disabled: true, mathml.legacy_number_syntax.disabled: true, mathml.mathsize_names.disabled:true, mathml.mathspace_names.disabled: true, mathml.mfrac_linethickness_names.disabled:true, mathml.nonzero_unitless_lengths.disabled:true, mathml.deprecated_alignment_attributes.disabled: true]
+prefs: [mathml.deprecated_style_attributes.disabled: true, mathml.deprecated_menclose_notation_radical.disabled: true, mathml.legacy_number_syntax.disabled: true, mathml.mathsize_names.disabled:true, mathml.mathspace_names.disabled: true, mathml.mfrac_linethickness_names.disabled:true, mathml.nonzero_unitless_lengths.disabled:true, mathml.deprecated_alignment_attributes.disabled: true]
\ No newline at end of file

deleted file mode 100644
--- a/testing/web-platform/meta/mathml/presentation-markup/menclose/legacy-menclose-radical-notation.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[legacy-menclose-radical-notation.html]
-  expected: FAIL

--- a/testing/web-platform/tests/mathml/presentation-markup/menclose/legacy-menclose-radical-notation-ref.html
+++ b/testing/web-platform/tests/mathml/presentation-markup/menclose/legacy-menclose-radical-notation-ref.html
@@ -10,11 +10,14 @@
       }
     </style>
   </head>
   <body>
     <math>
       <menclose>
         <mn>123</mn>
       </menclose>
+      <menclose notation="box">
+        <mn>123</mn>
+      </menclose>
     </math>
   </body>
 </html>

--- a/testing/web-platform/tests/mathml/presentation-markup/menclose/legacy-menclose-radical-notation.html
+++ b/testing/web-platform/tests/mathml/presentation-markup/menclose/legacy-menclose-radical-notation.html
@@ -1,27 +1,30 @@
 <!DOCTYPE html>
 <html>
   <head>
     <meta charset="utf-8"/>
     <title>Legacy menclose radical notation</title>
     <link rel="help" href="https://www.w3.org/TR/MathML3/chapter3.html#presm.menclose">
     <link rel="help" href="https://mathml-refresh.github.io/mathml-core/#enclose-expression-inside-notation-menclose">
-    <meta name="assert" content="Verify that the legacy menclose radical notation is not supported.">
+    <meta name="assert" content="Verify that the legacy menclose radical notation is ignored.">
     <link rel="match" href="legacy-menclose-radical-notation-ref.html">
     <link rel="stylesheet" type="text/css" href="/fonts/ahem.css" />
     <style>
       math, math * {
           font: 25px Ahem;
       }
     </style>
   </head>
   <body>
     <math>
       <menclose notation="radical">
         <mn>123</mn>
       </menclose>
+      <menclose notation="box radical">
+        <mn>123</mn>
+      </menclose>
     </math>
 
    <script src="/mathml/support/feature-detection.js"></script>
    <script>MathMLFeatureDetection.ensure_for_match_reftest("has_menclose");</script>
   </body>
 </html>