diff --git a/js/public/Utility.h b/js/public/Utility.h
index 14e29dd41..a9d889df1 100644
--- a/js/public/Utility.h
+++ b/js/public/Utility.h
@@ -68,6 +68,7 @@ enum ThreadType {
     THREAD_TYPE_GCHELPER,       // 6
     THREAD_TYPE_GCPARALLEL,     // 7
     THREAD_TYPE_PROMISE_TASK,   // 8
+    THREAD_TYPE_ION_FREE,       // 9
     THREAD_TYPE_MAX             // Used to check shell function arguments
 };
 
diff --git a/js/src/jit/Ion.cpp b/js/src/jit/Ion.cpp
index 01ea5b20b..077d891ff 100644
--- a/js/src/jit/Ion.cpp
+++ b/js/src/jit/Ion.cpp
@@ -474,6 +474,17 @@ JitCompartment::ensureIonStubsExist(JSContext* cx)
     return true;
 }
 
+void
+jit::FreeIonBuilder(IonBuilder* builder)
+{
+    // The builder is allocated into its LifoAlloc, so destroying that will
+    // destroy the builder and all other data accumulated during compilation,
+    // except any final codegen (which includes an assembler and needs to be
+    // explicitly destroyed).
+    js_delete(builder->backgroundCodegen());
+    js_delete(builder->alloc().lifoAlloc());
+}
+
 void
 jit::FinishOffThreadBuilder(JSRuntime* runtime, IonBuilder* builder,
                             const AutoLockHelperThreadState& locked)
@@ -505,12 +516,9 @@ jit::FinishOffThreadBuilder(JSRuntime* runtime, IonBuilder* builder,
         builder->script()->setIonScript(runtime, ion);
     }
 
-    // The builder is allocated into its LifoAlloc, so destroying that will
-    // destroy the builder and all other data accumulated during compilation,
-    // except any final codegen (which includes an assembler and needs to be
-    // explicitly destroyed).
-    js_delete(builder->backgroundCodegen());
-    js_delete(builder->alloc().lifoAlloc());
+    // Free Ion LifoAlloc off-thread. Free on the main thread if this OOMs.
+    if (!StartOffThreadIonFree(builder, locked))
+        FreeIonBuilder(builder);
 }
 
 static bool
diff --git a/js/src/jit/Ion.h b/js/src/jit/Ion.h
index bff01c944..44c205864 100644
--- a/js/src/jit/Ion.h
+++ b/js/src/jit/Ion.h
@@ -171,6 +171,7 @@ CodeGenerator* CompileBackEnd(MIRGenerator* mir);
 void AttachFinishedCompilations(JSContext* cx);
 void FinishOffThreadBuilder(JSRuntime* runtime, IonBuilder* builder,
                             const AutoLockHelperThreadState& lock);
+void FreeIonBuilder(IonBuilder* builder);
 
 void LinkIonScript(JSContext* cx, HandleScript calleescript);
 uint8_t* LazyLinkTopActivation(JSContext* cx);
diff --git a/js/src/vm/HelperThreads.cpp b/js/src/vm/HelperThreads.cpp
index a1cc7e75a..2ad09c716 100644
--- a/js/src/vm/HelperThreads.cpp
+++ b/js/src/vm/HelperThreads.cpp
@@ -110,6 +110,18 @@ js::StartOffThreadIonCompile(JSContext* cx, jit::IonBuilder* builder)
     return true;
 }
 
+bool
+js::StartOffThreadIonFree(jit::IonBuilder* builder, const AutoLockHelperThreadState& lock)
+{
+    MOZ_ASSERT(CanUseExtraThreads());
+
+    if (!HelperThreadState().ionFreeList(lock).append(builder))
+        return false;
+
+    HelperThreadState().notifyOne(GlobalHelperThreadState::PRODUCER, lock);
+    return true;
+}
+
 /*
  * Move an IonBuilder for which compilation has either finished, failed, or
  * been cancelled into the global finished compilation list. All off thread
@@ -784,6 +796,14 @@ void
 GlobalHelperThreadState::finish()
 {
     finishThreads();
+
+    // Make sure there are no Ion free tasks left. We check this here because,
+    // unlike the other tasks, we don't explicitly block on this when
+    // destroying a runtime.
+    AutoLockHelperThreadState lock;
+    auto& freeList = ionFreeList(lock);
+    while (!freeList.empty())
+        jit::FreeIonBuilder(freeList.popCopy());
 }
 
 void
@@ -994,6 +1014,12 @@ GlobalHelperThreadState::canStartIonCompile(const AutoLockHelperThreadState& loc
            checkTaskThreadLimit<jit::IonBuilder*>(maxIonCompilationThreads());
 }
 
+bool
+GlobalHelperThreadState::canStartIonFreeTask(const AutoLockHelperThreadState& lock)
+{
+    return !ionFreeList(lock).empty();
+}
+
 jit::IonBuilder*
 GlobalHelperThreadState::highestPriorityPendingIonCompile(const AutoLockHelperThreadState& lock,
                                                           bool remove /* = false */)
@@ -1610,6 +1636,21 @@ CurrentHelperThread()
     return thread;
 }
 
+void
+HelperThread::handleIonFreeWorkload(AutoLockHelperThreadState& locked)
+{
+    MOZ_ASSERT(idle());
+    MOZ_ASSERT(HelperThreadState().canStartIonFreeTask(locked));
+
+    auto& freeList = HelperThreadState().ionFreeList(locked);
+
+    jit::IonBuilder* builder = freeList.popCopy();
+    {
+        AutoUnlockHelperThreadState unlock(locked);
+        FreeIonBuilder(builder);
+    }
+}
+
 void
 js::PauseCurrentHelperThread()
 {
@@ -1925,6 +1966,8 @@ HelperThread::threadLoop()
                 task = js::oom::THREAD_TYPE_PARSE;
             else if (HelperThreadState().canStartCompressionTask(lock))
                 task = js::oom::THREAD_TYPE_COMPRESS;
+            else if (HelperThreadState().canStartIonFreeTask(lock))
+                task = js::oom::THREAD_TYPE_ION_FREE;
             else
                 task = js::oom::THREAD_TYPE_NONE;
 
@@ -1957,6 +2000,9 @@ HelperThread::threadLoop()
           case js::oom::THREAD_TYPE_COMPRESS:
             handleCompressionWorkload(lock);
             break;
+          case js::oom::THREAD_TYPE_ION_FREE:
+            handleIonFreeWorkload(lock);
+            break;
           default:
             MOZ_CRASH("No task to perform");
         }
diff --git a/js/src/vm/HelperThreads.h b/js/src/vm/HelperThreads.h
index 2c7d38769..ad7dcf3c5 100644
--- a/js/src/vm/HelperThreads.h
+++ b/js/src/vm/HelperThreads.h
@@ -84,7 +84,7 @@ class GlobalHelperThreadState
     // The lists below are all protected by |lock|.
 
     // Ion compilation worklist and finished jobs.
-    IonBuilderVector ionWorklist_, ionFinishedList_;
+    IonBuilderVector ionWorklist_, ionFinishedList_, ionFreeList_;
 
     // wasm worklist and finished jobs.
     wasm::CompileTaskPtrVector wasmWorklist_, wasmFinishedList_;
@@ -165,6 +165,9 @@ class GlobalHelperThreadState
     IonBuilderVector& ionFinishedList(const AutoLockHelperThreadState&) {
         return ionFinishedList_;
     }
+    IonBuilderVector& ionFreeList(const AutoLockHelperThreadState&) {
+        return ionFreeList_;
+    }
 
     wasm::CompileTaskPtrVector& wasmWorklist(const AutoLockHelperThreadState&) {
         return wasmWorklist_;
@@ -202,6 +205,7 @@ class GlobalHelperThreadState
     bool canStartWasmCompile(const AutoLockHelperThreadState& lock);
     bool canStartPromiseTask(const AutoLockHelperThreadState& lock);
     bool canStartIonCompile(const AutoLockHelperThreadState& lock);
+    bool canStartIonFreeTask(const AutoLockHelperThreadState& lock);
     bool canStartParseTask(const AutoLockHelperThreadState& lock);
     bool canStartCompressionTask(const AutoLockHelperThreadState& lock);
     bool canStartGCHelperTask(const AutoLockHelperThreadState& lock);
@@ -382,6 +386,7 @@ struct HelperThread
     void handleWasmWorkload(AutoLockHelperThreadState& locked);
     void handlePromiseTaskWorkload(AutoLockHelperThreadState& locked);
     void handleIonWorkload(AutoLockHelperThreadState& locked);
+    void handleIonFreeWorkload(AutoLockHelperThreadState& locked);
     void handleParseWorkload(AutoLockHelperThreadState& locked, uintptr_t stackLimit);
     void handleCompressionWorkload(AutoLockHelperThreadState& locked);
     void handleGCHelperWorkload(AutoLockHelperThreadState& locked);
@@ -439,6 +444,12 @@ StartPromiseTask(JSContext* cx, UniquePtr<PromiseTask> task);
 bool
 StartOffThreadIonCompile(JSContext* cx, jit::IonBuilder* builder);
 
+/*
+ * Schedule deletion of Ion compilation data.
+ */
+bool
+StartOffThreadIonFree(jit::IonBuilder* builder, const AutoLockHelperThreadState& lock);
+
 struct AllCompilations {};
 struct ZonesInState { JSRuntime* runtime; JS::Zone::GCState state; };