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diff --git a/toolkit/components/uniffi-js/ScaffoldingCall.h b/toolkit/components/uniffi-js/ScaffoldingCall.h
new file mode 100644
index 0000000000..02b016c513
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+++ b/toolkit/components/uniffi-js/ScaffoldingCall.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_ScaffoldingCall_h
+#define mozilla_ScaffoldingCall_h
+
+#include <tuple>
+#include <type_traits>
+#include "nsIGlobalObject.h"
+#include "nsPrintfCString.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/ResultVariant.h"
+#include "mozilla/dom/OwnedRustBuffer.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/ScaffoldingConverter.h"
+#include "mozilla/dom/UniFFIBinding.h"
+#include "mozilla/dom/UniFFIRust.h"
+
+namespace mozilla::uniffi {
+
+// Low-level result of calling a scaffolding function
+//
+// This stores what Rust returned in order to convert it into
+// UniFFIScaffoldingCallResult
+template <typename ReturnType>
+struct RustCallResult {
+  ReturnType mReturnValue;
+  RustCallStatus mCallStatus = {};
+};
+
+template <>
+struct RustCallResult<void> {
+  RustCallStatus mCallStatus = {};
+};
+
+// Does the work required to call a scaffolding function
+//
+// This class is generic over the type signature of the scaffolding function.
+// This seems better than being generic over the functions themselves, since it
+// saves space whenever 2 functions share a signature.
+template <typename ReturnConverter, typename... ArgConverters>
+class ScaffoldingCallHandler {
+ public:
+  // Pointer to a scaffolding function that can be called by this
+  // ScaffoldingConverter
+  using ScaffoldingFunc = typename ReturnConverter::RustType (*)(
+      typename ArgConverters::RustType..., RustCallStatus*);
+
+  // Perform an async scaffolding call
+  static already_AddRefed<dom::Promise> CallAsync(
+      ScaffoldingFunc aScaffoldingFunc, const dom::GlobalObject& aGlobal,
+      const dom::Sequence<dom::ScaffoldingType>& aArgs,
+      const nsLiteralCString& aFuncName, ErrorResult& aError) {
+    auto convertResult = ConvertJsArgs(aArgs);
+    if (convertResult.isErr()) {
+      aError.ThrowUnknownError(aFuncName + convertResult.unwrapErr());
+      return nullptr;
+    }
+    auto convertedArgs = convertResult.unwrap();
+
+    // Create the promise that we return to JS
+    nsCOMPtr<nsIGlobalObject> xpcomGlobal =
+        do_QueryInterface(aGlobal.GetAsSupports());
+    RefPtr<dom::Promise> returnPromise =
+        dom::Promise::Create(xpcomGlobal, aError);
+    if (aError.Failed()) {
+      return nullptr;
+    }
+
+    // Create a second promise that gets resolved by a background task that
+    // calls the scaffolding function
+    RefPtr taskPromise = new typename TaskPromiseType::Private(aFuncName.get());
+    nsresult dispatchResult = NS_DispatchBackgroundTask(
+        NS_NewRunnableFunction(aFuncName.get(),
+                               [args = std::move(convertedArgs), taskPromise,
+                                aScaffoldingFunc, aFuncName]() mutable {
+                                 auto callResult = CallScaffoldingFunc(
+                                     aScaffoldingFunc, std::move(args));
+                                 taskPromise->Resolve(std::move(callResult),
+                                                      aFuncName.get());
+                               }),
+        NS_DISPATCH_EVENT_MAY_BLOCK);
+    if (NS_FAILED(dispatchResult)) {
+      taskPromise->Reject(dispatchResult, aFuncName.get());
+    }
+
+    // When the background task promise completes, resolve the JS promise
+    taskPromise->Then(
+        GetCurrentSerialEventTarget(), aFuncName.get(),
+        [xpcomGlobal, returnPromise,
+         aFuncName](typename TaskPromiseType::ResolveOrRejectValue&& aResult) {
+          if (!aResult.IsResolve()) {
+            returnPromise->MaybeRejectWithUnknownError(aFuncName);
+            return;
+          }
+
+          dom::AutoEntryScript aes(xpcomGlobal, aFuncName.get());
+          dom::RootedDictionary<dom::UniFFIScaffoldingCallResult> returnValue(
+              aes.cx());
+
+          ReturnResult(aes.cx(), aResult.ResolveValue(), returnValue,
+                       aFuncName);
+          returnPromise->MaybeResolve(returnValue);
+        });
+
+    // Return the JS promise, using forget() to convert it to already_AddRefed
+    return returnPromise.forget();
+  }
+
+  // Perform an sync scaffolding call
+  //
+  // aFuncName should be a literal C string
+  static void CallSync(
+      ScaffoldingFunc aScaffoldingFunc, const dom::GlobalObject& aGlobal,
+      const dom::Sequence<dom::ScaffoldingType>& aArgs,
+      dom::RootedDictionary<dom::UniFFIScaffoldingCallResult>& aReturnValue,
+      const nsLiteralCString& aFuncName, ErrorResult& aError) {
+    auto convertResult = ConvertJsArgs(aArgs);
+    if (convertResult.isErr()) {
+      aError.ThrowUnknownError(aFuncName + convertResult.unwrapErr());
+      return;
+    }
+
+    auto callResult = CallScaffoldingFunc(aScaffoldingFunc,
+                                          std::move(convertResult.unwrap()));
+
+    ReturnResult(aGlobal.Context(), callResult, aReturnValue, aFuncName);
+  }
+
+ private:
+  using RustArgs = std::tuple<typename ArgConverters::RustType...>;
+  using IntermediateArgs =
+      std::tuple<typename ArgConverters::IntermediateType...>;
+  using CallResult = RustCallResult<typename ReturnConverter::RustType>;
+  using TaskPromiseType = MozPromise<CallResult, nsresult, true>;
+
+  template <size_t I>
+  using NthArgConverter =
+      typename std::tuple_element<I, std::tuple<ArgConverters...>>::type;
+
+  // Convert arguments from JS
+  //
+  // This should be called in the main thread
+  static Result<IntermediateArgs, nsCString> ConvertJsArgs(
+      const dom::Sequence<dom::ScaffoldingType>& aArgs) {
+    IntermediateArgs convertedArgs;
+    if (aArgs.Length() != std::tuple_size_v<IntermediateArgs>) {
+      return mozilla::Err("Wrong argument count"_ns);
+    }
+    auto result = PrepareArgsHelper<0>(aArgs, convertedArgs);
+    return result.map([&](auto _) { return std::move(convertedArgs); });
+  }
+
+  // Helper function for PrepareArgs that uses c++ magic to help with iteration
+  template <size_t I = 0>
+  static Result<mozilla::Ok, nsCString> PrepareArgsHelper(
+      const dom::Sequence<dom::ScaffoldingType>& aArgs,
+      IntermediateArgs& aConvertedArgs) {
+    if constexpr (I >= sizeof...(ArgConverters)) {
+      // Iteration complete
+      return mozilla::Ok();
+    } else {
+      // Single iteration step
+      auto result = NthArgConverter<I>::FromJs(aArgs[I]);
+      if (result.isOk()) {
+        // The conversion worked, store our result and move on to the next
+        std::get<I>(aConvertedArgs) = result.unwrap();
+        return PrepareArgsHelper<I + 1>(aArgs, aConvertedArgs);
+      } else {
+        // The conversion failed, return an error and don't continue
+        return mozilla::Err(result.unwrapErr() +
+                            nsPrintfCString(" (arg %zu)", I));
+      }
+    }
+  }
+
+  // Call the scaffolding function
+  //
+  // For async calls this should be called in the worker thread
+  static CallResult CallScaffoldingFunc(ScaffoldingFunc aFunc,
+                                        IntermediateArgs&& aArgs) {
+    return CallScaffoldingFuncHelper(
+        aFunc, std::move(aArgs), std::index_sequence_for<ArgConverters...>());
+  }
+
+  // Helper function for CallScaffoldingFunc that uses c++ magic to help with
+  // iteration
+  template <size_t... Is>
+  static CallResult CallScaffoldingFuncHelper(ScaffoldingFunc aFunc,
+                                              IntermediateArgs&& aArgs,
+                                              std::index_sequence<Is...> seq) {
+    CallResult result;
+
+    auto makeCall = [&]() mutable {
+      return aFunc(
+          NthArgConverter<Is>::IntoRust(std::move(std::get<Is>(aArgs)))...,
+          &result.mCallStatus);
+    };
+    if constexpr (std::is_void_v<typename ReturnConverter::RustType>) {
+      makeCall();
+    } else {
+      result.mReturnValue = makeCall();
+    }
+    return result;
+  }
+
+  // Return the result of the scaffolding call back to JS
+  //
+  // This should be called on the main thread
+  static void ReturnResult(
+      JSContext* aContext, CallResult& aCallResult,
+      dom::RootedDictionary<dom::UniFFIScaffoldingCallResult>& aReturnValue,
+      const nsLiteralCString& aFuncName) {
+    switch (aCallResult.mCallStatus.code) {
+      case RUST_CALL_SUCCESS: {
+        aReturnValue.mCode = dom::UniFFIScaffoldingCallCode::Success;
+        if constexpr (!std::is_void_v<typename ReturnConverter::RustType>) {
+          auto convertResult =
+              ReturnConverter::FromRust(aCallResult.mReturnValue);
+          if (convertResult.isOk()) {
+            ReturnConverter::IntoJs(aContext, std::move(convertResult.unwrap()),
+                                    aReturnValue.mData.Construct());
+          } else {
+            aReturnValue.mCode = dom::UniFFIScaffoldingCallCode::Internal_error;
+            aReturnValue.mInternalErrorMessage.Construct(
+                aFuncName + " converting result: "_ns +
+                convertResult.unwrapErr());
+          }
+        }
+        break;
+      }
+
+      case RUST_CALL_ERROR: {
+        // Rust Err() value.  Populate data with the `RustBuffer` containing the
+        // error
+        aReturnValue.mCode = dom::UniFFIScaffoldingCallCode::Error;
+        aReturnValue.mData.Construct().SetAsArrayBuffer().Init(
+            OwnedRustBuffer(aCallResult.mCallStatus.error_buf)
+                .IntoArrayBuffer(aContext));
+        break;
+      }
+
+      default: {
+        // This indicates a RustError, which shouldn't happen in practice since
+        // FF sets panic=abort
+        aReturnValue.mCode = dom::UniFFIScaffoldingCallCode::Internal_error;
+        aReturnValue.mInternalErrorMessage.Construct(aFuncName +
+                                                     " Unexpected Error"_ns);
+        break;
+      }
+    }
+  }
+};
+
+}  // namespace mozilla::uniffi
+
+#endif  // mozilla_ScaffoldingCall_h