diff --git a/js/src/frontend/BytecodeEmitter.cpp b/js/src/frontend/BytecodeEmitter.cpp
index 367fea8ca5e0..155f8ed1805d 100644
--- a/js/src/frontend/BytecodeEmitter.cpp
+++ b/js/src/frontend/BytecodeEmitter.cpp
@@ -5138,8 +5138,9 @@ EmitFunc(JSContext *cx, BytecodeEmitter *bce, ParseNode *pn)
             return false;
         if (pn->pn_cookie.isFree()) {
             bce->switchToProlog();
-            JSOp op = fun->isFlatClosure() ? JSOP_DEFFUN_FC : JSOP_DEFFUN;
-            if (!EmitFunctionOp(cx, op, index, bce))
+            MOZ_ASSERT(!fun->isFlatClosure(),
+                       "global functions can't have upvars, so they are never flat");
+            if (!EmitFunctionOp(cx, JSOP_DEFFUN, index, bce))
                 return false;
             bce->switchToMain();
         }
diff --git a/js/src/frontend/SemanticAnalysis.cpp b/js/src/frontend/SemanticAnalysis.cpp
index 09c6a0651307..afbba44a0cb4 100644
--- a/js/src/frontend/SemanticAnalysis.cpp
+++ b/js/src/frontend/SemanticAnalysis.cpp
@@ -570,15 +570,20 @@ SetFunctionKinds(FunctionBox *funbox, uint32_t *tcflags, bool isDirectEval)
                 }
             }
 
+            /*
+             * Top-level functions, and (extension) functions not at top level
+             * which are also not directly within other functions, aren't
+             * flattened.
+             */
+            if (fn->isOp(JSOP_DEFFUN))
+                canFlatten = false;
+
             if (!hasUpvars) {
                 /* No lexical dependencies => null closure, for best performance. */
                 fun->setKind(JSFUN_NULL_CLOSURE);
             } else if (canFlatten) {
                 fun->setKind(JSFUN_FLAT_CLOSURE);
                 switch (fn->getOp()) {
-                  case JSOP_DEFFUN:
-                    fn->setOp(JSOP_DEFFUN_FC);
-                    break;
                   case JSOP_DEFLOCALFUN:
                     fn->setOp(JSOP_DEFLOCALFUN_FC);
                     break;
diff --git a/js/src/jsinfer.cpp b/js/src/jsinfer.cpp
index aefc1887bdc9..6c49497da9e1 100644
--- a/js/src/jsinfer.cpp
+++ b/js/src/jsinfer.cpp
@@ -3822,7 +3822,6 @@ ScriptAnalysis::analyzeTypesBytecode(JSContext *cx, unsigned offset,
       case JSOP_LAMBDA:
       case JSOP_LAMBDA_FC:
       case JSOP_DEFFUN:
-      case JSOP_DEFFUN_FC:
       case JSOP_DEFLOCALFUN:
       case JSOP_DEFLOCALFUN_FC: {
         unsigned off = (op == JSOP_DEFLOCALFUN || op == JSOP_DEFLOCALFUN_FC) ? SLOTNO_LEN : 0;
diff --git a/js/src/jsinterp.cpp b/js/src/jsinterp.cpp
index f87c8ddc075a..d2334ca7ee4f 100644
--- a/js/src/jsinterp.cpp
+++ b/js/src/jsinterp.cpp
@@ -1744,6 +1744,7 @@ ADD_EMPTY_CASE(JSOP_UNUSED13)
 ADD_EMPTY_CASE(JSOP_UNUSED14)
 ADD_EMPTY_CASE(JSOP_UNUSED15)
 ADD_EMPTY_CASE(JSOP_UNUSED16)
+ADD_EMPTY_CASE(JSOP_UNUSED17)
 ADD_EMPTY_CASE(JSOP_CONDSWITCH)
 ADD_EMPTY_CASE(JSOP_TRY)
 #if JS_HAS_XML_SUPPORT
@@ -3268,35 +3269,6 @@ BEGIN_CASE(JSOP_DEFFUN)
 }
 END_CASE(JSOP_DEFFUN)
 
-BEGIN_CASE(JSOP_DEFFUN_FC)
-{
-    JSFunction *fun = script->getFunction(GET_UINT32_INDEX(regs.pc));
-
-    JSObject *obj = js_NewFlatClosure(cx, fun);
-    if (!obj)
-        goto error;
-
-    Value rval = ObjectValue(*obj);
-
-    uintN attrs = regs.fp()->isEvalFrame()
-                  ? JSPROP_ENUMERATE
-                  : JSPROP_ENUMERATE | JSPROP_PERMANENT;
-
-    JSObject &parent = regs.fp()->varObj();
-
-    PropertyName *name = fun->atom->asPropertyName();
-    if (!CheckRedeclaration(cx, &parent, name, attrs))
-        goto error;
-
-    if ((attrs == JSPROP_ENUMERATE)
-        ? !parent.setProperty(cx, name, &rval, script->strictModeCode)
-        : !parent.defineProperty(cx, name, rval, JS_PropertyStub, JS_StrictPropertyStub, attrs))
-    {
-        goto error;
-    }
-}
-END_CASE(JSOP_DEFFUN_FC)
-
 BEGIN_CASE(JSOP_DEFLOCALFUN)
 {
     /*
diff --git a/js/src/jsopcode.cpp b/js/src/jsopcode.cpp
index ba32fb133e8c..dcd05a1511b7 100644
--- a/js/src/jsopcode.cpp
+++ b/js/src/jsopcode.cpp
@@ -5133,11 +5133,9 @@ Decompile(SprintStack *ss, jsbytecode *pc, intN nb)
               }
 
               case JSOP_DEFFUN:
-              case JSOP_DEFFUN_FC:
                 fun = jp->script->getFunction(GET_UINT32_INDEX(pc));
                 todo = -2;
                 goto do_function;
-                break;
 
               case JSOP_HOLE:
                 todo = SprintPut(&ss->sprinter, "", 0);
diff --git a/js/src/jsopcode.tbl b/js/src/jsopcode.tbl
index 6e24ac30408b..534e59f226fd 100644
--- a/js/src/jsopcode.tbl
+++ b/js/src/jsopcode.tbl
@@ -552,10 +552,11 @@ OPDEF(JSOP_LENGTH,        217, "length",       NULL,  3,  1,  1, 18,  JOF_ATOM|J
  */
 OPDEF(JSOP_HOLE,          218, "hole",         NULL,  1,  0,  1,  0,  JOF_BYTE)
 
+OPDEF(JSOP_UNUSED17,      219,"unused17",      NULL,  1,  0,  0,  0,  JOF_BYTE)
+
 /*
- * Variants of JSOP_{DEF{,LOCAL}FUN,LAMBDA} optimized for the flat closure case.
+ * Variants of JSOP_{DEFLOCALFUN,LAMBDA} optimized for the flat closure case.
  */
-OPDEF(JSOP_DEFFUN_FC,     219,"deffun_fc",     NULL,  5,  0,  0,  0,  JOF_OBJECT|JOF_DECLARING)
 OPDEF(JSOP_DEFLOCALFUN_FC,220,"deflocalfun_fc",NULL,  7,  0,  0,  0,  JOF_SLOTOBJECT|JOF_DECLARING|JOF_TMPSLOT)
 OPDEF(JSOP_LAMBDA_FC,     221,"lambda_fc",     NULL,  5,  0,  1, 19,  JOF_OBJECT)