--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -13555,17 +13555,19 @@ class CGBindingRoot(CGThing):
     def deps(self):
         return self.root.deps()
 
 
 class CGNativeMember(ClassMethod):
     def __init__(self, descriptorProvider, member, name, signature, extendedAttrs,
                  breakAfter=True, passJSBitsAsNeeded=True, visibility="public",
                  typedArraysAreStructs=True, variadicIsSequence=False,
-                 resultNotAddRefed=False):
+                 resultNotAddRefed=False,
+                 virtual=False,
+                 override=False):
         """
         If typedArraysAreStructs is false, typed arrays will be passed as
         JS::Handle<JSObject*>.  If it's true they will be passed as one of the
         dom::TypedArray subclasses.
 
         If passJSBitsAsNeeded is false, we don't automatically pass in a
         JSContext* or a JSObject* based on the return and argument types.  We
         can still pass it based on 'implicitJSContext' annotations.
@@ -13583,17 +13585,19 @@ class CGNativeMember(ClassMethod):
                              self.getArgs(signature[0], signature[1]),
                              static=member.isStatic(),
                              # Mark our getters, which are attrs that
                              # have a non-void return type, as const.
                              const=(not member.isStatic() and member.isAttr() and
                                     not signature[0].isVoid()),
                              breakAfterReturnDecl=" ",
                              breakAfterSelf=breakAfterSelf,
-                             visibility=visibility)
+                             visibility=visibility,
+                             virtual=virtual,
+                             override=override)
 
     def getReturnType(self, type, isMember):
         return self.getRetvalInfo(type, isMember)[0]
 
     def getRetvalInfo(self, type, isMember):
         """
         Returns a tuple:
 
@@ -14360,47 +14364,59 @@ def jsImplName(name):
 
 class CGJSImplMember(CGNativeMember):
     """
     Base class for generating code for the members of the implementation class
     for a JS-implemented WebIDL interface.
     """
     def __init__(self, descriptorProvider, member, name, signature,
                  extendedAttrs, breakAfter=True, passJSBitsAsNeeded=True,
-                 visibility="public", variadicIsSequence=False):
+                 visibility="public", variadicIsSequence=False,
+                 virtual=False, override=False):
         CGNativeMember.__init__(self, descriptorProvider, member, name,
                                 signature, extendedAttrs, breakAfter=breakAfter,
                                 passJSBitsAsNeeded=passJSBitsAsNeeded,
                                 visibility=visibility,
-                                variadicIsSequence=variadicIsSequence)
+                                variadicIsSequence=variadicIsSequence,
+                                virtual=virtual,
+                                override=override)
         self.body = self.getImpl()
 
     def getArgs(self, returnType, argList):
         args = CGNativeMember.getArgs(self, returnType, argList)
         args.append(Argument("JSCompartment*", "aCompartment", "nullptr"))
         return args
 
 
 class CGJSImplMethod(CGJSImplMember):
     """
     Class for generating code for the methods for a JS-implemented WebIDL
     interface.
     """
     def __init__(self, descriptor, method, signature, isConstructor, breakAfter=True):
+        virtual = False
+        override = False
+        if (method.identifier.name == "eventListenerWasAdded" or
+            method.identifier.name == "eventListenerWasRemoved"):
+            virtual = True
+            override = True
+
         self.signature = signature
         self.descriptor = descriptor
         self.isConstructor = isConstructor
         CGJSImplMember.__init__(self, descriptor, method,
                                 CGSpecializedMethod.makeNativeName(descriptor,
                                                                    method),
                                 signature,
                                 descriptor.getExtendedAttributes(method),
                                 breakAfter=breakAfter,
                                 variadicIsSequence=True,
-                                passJSBitsAsNeeded=False)
+                                passJSBitsAsNeeded=False,
+                                virtual=virtual,
+                                override=override)
 
     def getArgs(self, returnType, argList):
         if self.isConstructor:
             # Skip the JSCompartment bits for constructors; it's handled
             # manually in getImpl.
             return CGNativeMember.getArgs(self, returnType, argList)
         return CGJSImplMember.getArgs(self, returnType, argList)