--- a/js/src/ion/Ion.h
+++ b/js/src/ion/Ion.h
@@ -83,17 +83,17 @@ struct IonOptions
 
     // Toggles whether Effective Address Analysis is performed.
     //
     // Default: true
     bool eaa;
 
     // Toggles whether compilation occurs off the main thread.
     //
-    // Default: true iff the build is JS_THREADSAFE.
+    // Default: true iff there are at least two CPUs available
     bool parallelCompilation;
 
     // How many invocations or loop iterations are needed before functions
     // are compiled with the baseline compiler.
     //
     // Default: 10
     uint32_t baselineUsesBeforeCompile;
 
@@ -197,21 +197,17 @@ struct IonOptions
         osr(true),
         limitScriptSize(true),
         registerAllocator(RegisterAllocator_LSRA),
         inlining(true),
         edgeCaseAnalysis(true),
         rangeAnalysis(true),
         uce(true),
         eaa(true),
-#ifdef JS_THREADSAFE
-        parallelCompilation(true),
-#else
         parallelCompilation(false),
-#endif
         baselineUsesBeforeCompile(10),
         usesBeforeCompile(1000),
         usesBeforeCompileNoJaeger(40),
         usesBeforeInliningFactor(.125),
         maxStackArgs(4096),
         maxInlineDepth(3),
         smallFunctionMaxInlineDepth(10),
         smallFunctionMaxBytecodeLength(100),