diff --git a/js/src/builtin/TestingFunctions.cpp b/js/src/builtin/TestingFunctions.cpp
index 6ffc1631593c..cb5a81ec913e 100644
--- a/js/src/builtin/TestingFunctions.cpp
+++ b/js/src/builtin/TestingFunctions.cpp
@@ -4963,6 +4963,24 @@ IsLegacyIterator(JSContext* cx, unsigned argc, Value* vp)
     return true;
 }
 
+static bool
+EnableExpressionClosures(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    JS::ContextOptionsRef(cx).setExpressionClosures(true);
+    args.rval().setUndefined();
+    return true;
+}
+
+static bool
+DisableExpressionClosures(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    JS::ContextOptionsRef(cx).setExpressionClosures(false);
+    args.rval().setUndefined();
+    return true;
+}
+
 static const JSFunctionSpecWithHelp TestingFunctions[] = {
     JS_FN_HELP("gc", ::GC, 0, 0,
 "gc([obj] | 'zone' [, 'shrinking'])",
@@ -5579,6 +5597,14 @@ gc::ZealModeHelpText),
 "getTimeZone()",
 "  Get the current time zone.\n"),
 
+    JS_FN_HELP("enableExpressionClosures", EnableExpressionClosures, 0, 0,
+"enableExpressionClosures()",
+"  Enables the deprecated, non-standard expression closures.\n"),
+
+    JS_FN_HELP("disableExpressionClosures", DisableExpressionClosures, 0, 0,
+"disableExpressionClosures()",
+"  Disables the deprecated, non-standard expression closures.\n"),
+
     JS_FS_HELP_END
 };
 
diff --git a/js/src/frontend/Parser.cpp b/js/src/frontend/Parser.cpp
index 79f681339e19..e8ae47e9dacf 100644
--- a/js/src/frontend/Parser.cpp
+++ b/js/src/frontend/Parser.cpp
@@ -3260,15 +3260,15 @@ Parser<FullParseHandler, CharT>::skipLazyInnerFunction(ParseNode* funcNode, uint
     if (!tokenStream.advance(fun->lazyScript()->end()))
         return false;
 
-#if JS_HAS_EXPR_CLOSURES
-    // Only expression closure can be Statement kind.
-    // If we remove expression closure, we can remove isExprBody flag from
-    // LazyScript and JSScript.
-    if (kind == Statement && funbox->isExprBody()) {
-        if (!matchOrInsertSemicolon())
-            return false;
+    if (allowExpressionClosures()) {
+        // Only expression closure can be Statement kind.
+        // If we remove expression closure, we can remove isExprBody flag from
+        // LazyScript and JSScript.
+        if (kind == Statement && funbox->isExprBody()) {
+            if (!matchOrInsertSemicolon())
+                return false;
+        }
     }
-#endif
 
     // Append possible Annex B function box only upon successfully parsing.
     if (tryAnnexB && !pc->innermostScope()->addPossibleAnnexBFunctionBox(pc, funbox))
@@ -3778,14 +3778,14 @@ GeneralParser<ParseHandler, CharT>::functionFormalParametersAndBody(InHandling i
                 return false;
             }
 
-#if JS_HAS_EXPR_CLOSURES
-            this->addTelemetry(DeprecatedLanguageExtension::ExpressionClosure);
-            if (!warnOnceAboutExprClosure())
+            if (allowExpressionClosures()) {
+                this->addTelemetry(DeprecatedLanguageExtension::ExpressionClosure);
+                if (!warnOnceAboutExprClosure())
+                    return false;
+            } else {
+                error(JSMSG_CURLY_BEFORE_BODY);
                 return false;
-#else
-            error(JSMSG_CURLY_BEFORE_BODY);
-            return false;
-#endif
+            }
         }
 
         anyChars.ungetToken();
@@ -3847,9 +3847,8 @@ GeneralParser<ParseHandler, CharT>::functionFormalParametersAndBody(InHandling i
                                                               JSMSG_CURLY_OPENED, openedPos));
         funbox->setEnd(anyChars);
     } else {
-#if !JS_HAS_EXPR_CLOSURES
-        MOZ_ASSERT(kind == Arrow);
-#endif
+        MOZ_ASSERT_IF(!allowExpressionClosures(), kind == Arrow);
+
         if (anyChars.hadError())
             return false;
         funbox->setEnd(anyChars);
diff --git a/js/src/frontend/Parser.h b/js/src/frontend/Parser.h
index 561ed3778c38..46d64ee999b2 100644
--- a/js/src/frontend/Parser.h
+++ b/js/src/frontend/Parser.h
@@ -340,7 +340,9 @@ class ParserBase
 
     bool hasValidSimpleStrictParameterNames();
 
-
+    bool allowExpressionClosures() const {
+        return options().expressionClosuresOption;
+    }
     /*
      * Create a new function object given a name (which is optional if this is
      * a function expression).
@@ -703,6 +705,7 @@ class GeneralParser
     using Base::isValidSimpleAssignmentTarget;
     using Base::pc;
     using Base::usedNames;
+    using Base::allowExpressionClosures;
 
   private:
     using Base::checkAndMarkSuperScope;
@@ -1403,6 +1406,7 @@ class Parser<FullParseHandler, CharT> final
     using Base::pos;
     using Base::ss;
     using Base::tokenStream;
+    using Base::allowExpressionClosures;
 
   private:
     using Base::alloc;
diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index 9f461beae4c9..c283bbe903a5 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -3991,6 +3991,7 @@ JS::TransitiveCompileOptions::copyPODTransitiveOptions(const TransitiveCompileOp
     canLazilyParse = rhs.canLazilyParse;
     strictOption = rhs.strictOption;
     extraWarningsOption = rhs.extraWarningsOption;
+    expressionClosuresOption = rhs.expressionClosuresOption;
     werrorOption = rhs.werrorOption;
     asmJSOption = rhs.asmJSOption;
     throwOnAsmJSValidationFailureOption = rhs.throwOnAsmJSValidationFailureOption;
@@ -4112,6 +4113,7 @@ JS::CompileOptions::CompileOptions(JSContext* cx)
 {
     strictOption = cx->options().strictMode();
     extraWarningsOption = cx->compartment()->behaviors().extraWarnings(cx);
+    expressionClosuresOption = cx->options().expressionClosures();
     isProbablySystemOrAddonCode = cx->compartment()->isProbablySystemOrAddonCode();
     werrorOption = cx->options().werror();
     if (!cx->options().asmJS())
diff --git a/js/src/jsapi.h b/js/src/jsapi.h
index 7a1200eced7e..16538dca1d28 100644
--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -1117,6 +1117,7 @@ class JS_PUBLIC_API(ContextOptions) {
 #ifdef FUZZING
         , fuzzing_(false)
 #endif
+        , expressionClosures_(false)
     {
     }
 
@@ -1272,6 +1273,12 @@ class JS_PUBLIC_API(ContextOptions) {
     }
 #endif
 
+    bool expressionClosures() const { return expressionClosures_; }
+    ContextOptions& setExpressionClosures(bool flag) {
+        expressionClosures_ = flag;
+        return *this;
+    }
+
     void disableOptionsForSafeMode() {
         setBaseline(false);
         setIon(false);
@@ -1302,6 +1309,7 @@ class JS_PUBLIC_API(ContextOptions) {
 #ifdef FUZZING
     bool fuzzing_ : 1;
 #endif
+    bool expressionClosures_ : 1;
 
 };
 
@@ -3668,6 +3676,7 @@ class JS_FRIEND_API(TransitiveCompileOptions)
         canLazilyParse(true),
         strictOption(false),
         extraWarningsOption(false),
+        expressionClosuresOption(false),
         werrorOption(false),
         asmJSOption(AsmJSOption::Disabled),
         throwOnAsmJSValidationFailureOption(false),
@@ -3702,6 +3711,7 @@ class JS_FRIEND_API(TransitiveCompileOptions)
     bool canLazilyParse;
     bool strictOption;
     bool extraWarningsOption;
+    bool expressionClosuresOption;
     bool werrorOption;
     AsmJSOption asmJSOption;
     bool throwOnAsmJSValidationFailureOption;
diff --git a/js/src/jsversion.h b/js/src/jsversion.h
index 82343696adca..061a50e1eafe 100644
--- a/js/src/jsversion.h
+++ b/js/src/jsversion.h
@@ -14,10 +14,6 @@
 #define JS_HAS_TOSOURCE         1       /* has Object/Array toSource method */
 #define JS_HAS_UNEVAL           1       /* has uneval() top-level function */
 
-#ifndef NIGHTLY_BUILD
-#define JS_HAS_EXPR_CLOSURES    1       /* has function (formals) listexpr */
-#endif
-
 /*
  * Feature for Object.prototype.__{define,lookup}{G,S}etter__ legacy support;
  * support likely to be made opt-in at some future time.