diff --git a/js/public/HeapAPI.h b/js/public/HeapAPI.h
index d9fcb99c032e..bccc4925d287 100644
--- a/js/public/HeapAPI.h
+++ b/js/public/HeapAPI.h
@@ -100,6 +100,12 @@ struct Zone;
 /* Default size for the generational nursery in bytes. */
 const uint32_t DefaultNurseryBytes = 16 * 1024 * 1024;
 
+/* Default maximum heap size in bytes to pass to JS_NewRuntime(). */
+const uint32_t DefaultHeapMaxBytes = 32 * 1024 * 1024;
+
+/* Value indicating no maximum heap size to pass to JS_NewRuntime(). */
+const uint32_t UnlimitedHeapMaxBytes = 0xffffffff;
+
 /*
  * We cannot expose the class hierarchy: the implementation is hidden. Instead
  * we provide cast functions with strong debug-mode assertions.
diff --git a/js/src/shell/js.cpp b/js/src/shell/js.cpp
index 76f1fb706461..46a798292685 100644
--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -6419,7 +6419,7 @@ main(int argc, char **argv, char **envp)
 #endif
 
     /* Use the same parameters as the browser in xpcjsruntime.cpp. */
-    rt = JS_NewRuntime(32L * 1024L * 1024L, nurseryBytes);
+    rt = JS_NewRuntime(JS::UnlimitedHeapMaxBytes, nurseryBytes);
     if (!rt)
         return 1;
 
@@ -6429,7 +6429,6 @@ main(int argc, char **argv, char **envp)
 
     gInterruptFunc.construct(rt, NullValue());
 
-    JS_SetGCParameter(rt, JSGC_MAX_BYTES, 0xffffffff);
 #ifdef JSGC_GENERATIONAL
     Maybe<JS::AutoDisableGenerationalGC> noggc;
     if (op.getBoolOption("no-ggc"))
diff --git a/js/xpconnect/src/XPCJSRuntime.cpp b/js/xpconnect/src/XPCJSRuntime.cpp
index f25db8e86c1c..ae244224d75d 100644
--- a/js/xpconnect/src/XPCJSRuntime.cpp
+++ b/js/xpconnect/src/XPCJSRuntime.cpp
@@ -3069,7 +3069,7 @@ static const JSWrapObjectCallbacks WrapObjectCallbacks = {
 };
 
 XPCJSRuntime::XPCJSRuntime(nsXPConnect* aXPConnect)
-   : CycleCollectedJSRuntime(nullptr, 32L * 1024L * 1024L),
+   : CycleCollectedJSRuntime(nullptr, JS::UnlimitedHeapMaxBytes),
    mJSContextStack(new XPCJSContextStack(MOZ_THIS_IN_INITIALIZER_LIST())),
    mCallContext(nullptr),
    mAutoRoots(nullptr),
@@ -3108,14 +3108,6 @@ XPCJSRuntime::XPCJSRuntime(nsXPConnect* aXPConnect)
     memset(rtPrivate, 0, sizeof(PerThreadAtomCache));
     JS_SetRuntimePrivate(runtime, rtPrivate);
 
-    // Unconstrain the runtime's threshold on nominal heap size, to avoid
-    // triggering GC too often if operating continuously near an arbitrary
-    // finite threshold (0xffffffff is infinity for uint32_t parameters).
-    // This leaves the maximum-JS_malloc-bytes threshold still in effect
-    // to cause period, and we hope hygienic, last-ditch GCs from within
-    // the GC's allocator.
-    JS_SetGCParameter(runtime, JSGC_MAX_BYTES, 0xffffffff);
-
     // The JS engine permits us to set different stack limits for system code,
     // trusted script, and untrusted script. We have tests that ensure that
     // we can always execute 10 "heavy" (eval+with) stack frames deeper in