--- a/dom/tests/mochitest/general/test_interfaces.js
+++ b/dom/tests/mochitest/general/test_interfaces.js
@@ -69,17 +69,17 @@ var ecmaGlobals =
     "TypeError",
     "Uint16Array",
     "Uint32Array",
     "Uint8Array",
     "Uint8ClampedArray",
     "URIError",
     "WeakMap",
     "WeakSet",
-    "WebAssembly"
+    {name: "WebAssembly", disabled: !SpecialPowers.Cu.getJSTestingFunctions().wasmIsSupported()}
   ];
 // IMPORTANT: Do not change the list above without review from
 //            a JavaScript Engine peer!
 
 // IMPORTANT: Do not change the list below without review from a DOM peer,
 //            except to remove items from it!
 //
 // This is a list of interfaces that were prefixed with 'moz' instead of 'Moz'.

--- a/dom/workers/test/serviceworkers/test_serviceworker_interfaces.js
+++ b/dom/workers/test/serviceworkers/test_serviceworker_interfaces.js
@@ -64,17 +64,17 @@ var ecmaGlobals =
     "TypeError",
     "Uint16Array",
     "Uint32Array",
     "Uint8Array",
     "Uint8ClampedArray",
     "URIError",
     "WeakMap",
     "WeakSet",
-    "WebAssembly"
+    {name: "WebAssembly", optional: true}
   ];
 // IMPORTANT: Do not change the list above without review from
 //            a JavaScript Engine peer!
 
 // IMPORTANT: Do not change the list below without review from a DOM peer!
 var interfaceNamesInGlobalScope =
   [
 // IMPORTANT: Do not change this list without review from a DOM peer!
@@ -248,16 +248,18 @@ function createInterfaceMap(version, use
         ok(!("pref" in entry), "Bogus pref annotation for " + entry.name);
         if ((entry.nightly === !isNightly) ||
             (entry.nightlyAndroid === !(isAndroid && isNightly) && isAndroid) ||
             (entry.nonReleaseAndroid === !(isAndroid && !isRelease) && isAndroid) ||
             (entry.desktop === !isDesktop) ||
             (entry.android === !isAndroid && !entry.nonReleaseAndroid && !entry.nightlyAndroid) ||
             (entry.release === !isRelease)) {
           interfaceMap[entry.name] = false;
+        } else if (entry.optional) {
+          interfaceMap[entry.name] = "optional";
         } else {
           interfaceMap[entry.name] = true;
         }
       }
     }
   }
 
   addInterfaces(ecmaGlobals);
@@ -268,27 +270,31 @@ function createInterfaceMap(version, use
 
 function runTest(version, userAgent) {
   var interfaceMap = createInterfaceMap(version, userAgent);
   for (var name of Object.getOwnPropertyNames(self)) {
     // An interface name should start with an upper case character.
     if (!/^[A-Z]/.test(name)) {
       continue;
     }
-    ok(interfaceMap[name],
+    ok(interfaceMap[name] === "optional" || interfaceMap[name],
        "If this is failing: DANGER, are you sure you want to expose the new interface " + name +
        " to all webpages as a property on the service worker? Do not make a change to this file without a " +
        " review from a DOM peer for that specific change!!! (or a JS peer for changes to ecmaGlobals)");
     delete interfaceMap[name];
   }
   for (var name of Object.keys(interfaceMap)) {
-    ok(name in self === interfaceMap[name],
-       name + " should " + (interfaceMap[name] ? "" : " NOT") + " be defined on the global scope");
-    if (!interfaceMap[name]) {
+    if (interfaceMap[name] === "optional") {
       delete interfaceMap[name];
+    } else {
+      ok(name in self === interfaceMap[name],
+         name + " should " + (interfaceMap[name] ? "" : " NOT") + " be defined on the global scope");
+      if (!interfaceMap[name]) {
+        delete interfaceMap[name];
+      }
     }
   }
   is(Object.keys(interfaceMap).length, 0,
      "The following interface(s) are not enumerated: " + Object.keys(interfaceMap).join(", "));
 }
 
 workerTestGetVersion(function(version) {
   workerTestGetUserAgent(function(userAgent) {

--- a/dom/workers/test/test_worker_interfaces.js
+++ b/dom/workers/test/test_worker_interfaces.js
@@ -64,17 +64,17 @@ var ecmaGlobals =
     "TypeError",
     "Uint16Array",
     "Uint32Array",
     "Uint8Array",
     "Uint8ClampedArray",
     "URIError",
     "WeakMap",
     "WeakSet",
-    "WebAssembly"
+    {name: "WebAssembly", optional: true}
   ];
 // IMPORTANT: Do not change the list above without review from
 //            a JavaScript Engine peer!
 
 // IMPORTANT: Do not change the list below without review from a DOM peer!
 var interfaceNamesInGlobalScope =
   [
 // IMPORTANT: Do not change this list without review from a DOM peer!
@@ -262,16 +262,18 @@ function createInterfaceMap(version, use
         ok(!("pref" in entry), "Bogus pref annotation for " + entry.name);
         if ((entry.nightly === !isNightly) ||
             (entry.nightlyAndroid === !(isAndroid && isNightly) && isAndroid) ||
             (entry.desktop === !isDesktop) ||
             (entry.android === !isAndroid && !entry.nightlyAndroid) ||
             (entry.release === !isRelease) ||
             entry.disabled) {
           interfaceMap[entry.name] = false;
+        } else if (entry.optional) {
+          interfaceMap[entry.name] = "optional";
         } else {
           interfaceMap[entry.name] = true;
         }
       }
     }
   }
 
   addInterfaces(ecmaGlobals);
@@ -282,27 +284,31 @@ function createInterfaceMap(version, use
 
 function runTest(version, userAgent) {
   var interfaceMap = createInterfaceMap(version, userAgent);
   for (var name of Object.getOwnPropertyNames(self)) {
     // An interface name should start with an upper case character.
     if (!/^[A-Z]/.test(name)) {
       continue;
     }
-    ok(interfaceMap[name],
+    ok(interfaceMap[name] === "optional" || interfaceMap[name],
        "If this is failing: DANGER, are you sure you want to expose the new interface " + name +
        " to all webpages as a property on the worker? Do not make a change to this file without a " +
        " review from a DOM peer for that specific change!!! (or a JS peer for changes to ecmaGlobals)");
     delete interfaceMap[name];
   }
   for (var name of Object.keys(interfaceMap)) {
-    ok(name in self === interfaceMap[name],
-       name + " should " + (interfaceMap[name] ? "" : " NOT") + " be defined on the global scope");
-    if (!interfaceMap[name]) {
+    if (interfaceMap[name] === "optional") {
       delete interfaceMap[name];
+    } else {
+      ok(name in self === interfaceMap[name],
+         name + " should " + (interfaceMap[name] ? "" : " NOT") + " be defined on the global scope");
+      if (!interfaceMap[name]) {
+        delete interfaceMap[name];
+      }
     }
   }
   is(Object.keys(interfaceMap).length, 0,
      "The following interface(s) are not enumerated: " + Object.keys(interfaceMap).join(", "));
 }
 
 workerTestGetVersion(function(version) {
   workerTestGetUserAgent(function(userAgent) {