diff --git a/src/backends/wasm_js.rs b/src/backends/wasm_js.rs
index 61198f53..5feb0f3b 100644
--- a/src/backends/wasm_js.rs
+++ b/src/backends/wasm_js.rs
@@ -16,7 +16,7 @@ pub use crate::util::{inner_u32, inner_u64};
 #[cfg(not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "none"))))]
 compile_error!("`wasm_js` backend can be enabled only for OS-less WASM targets!");
 
-use js_sys::{SharedArrayBuffer, Uint8Array, WebAssembly::Memory};
+use js_sys::{JsString, Object, Uint8Array, WebAssembly::Memory};
 use wasm_bindgen::{prelude::wasm_bindgen, JsCast, JsValue};
 
 // Size of our temporary Uint8Array buffer used with WebCrypto methods
@@ -39,9 +39,15 @@ pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
                 MEMORY_KIND_SHARED => true,
                 MEMORY_KIND_UNINIT => {
                     let memory: Memory = wasm_bindgen::memory().unchecked_into();
-                    let val = if memory.buffer().is_instance_of::<SharedArrayBuffer>() {
+                    // We can't use `instanceof` because of a bug in Firefox.
+                    // Instead we compare the constructor name, which always works.
+                    let constructor_name = Object::from(memory.buffer()).constructor().name();
+                    let val = if SHARED_ARRAY_BUFFER_NAME.with(|name| &constructor_name == name) {
                         MEMORY_KIND_SHARED
                     } else {
+                        web_sys::console::log_1(
+                            &Object::from(memory.buffer()).constructor().name(),
+                        );
                         MEMORY_KIND_NOT_SHARED
                     };
                     MEMORY_KIND.store(val, Ordering::Relaxed);
@@ -87,6 +93,7 @@ pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
 }
 
 #[wasm_bindgen]
+#[rustfmt::skip]
 extern "C" {
     // Web Crypto API: Crypto interface (https://www.w3.org/TR/WebCryptoAPI/)
     type Crypto;
@@ -98,4 +105,6 @@ extern "C" {
     fn get_random_values(this: &Crypto, buf: &Uint8Array) -> Result<(), JsValue>;
     #[wasm_bindgen(method, js_name = getRandomValues, catch)]
     fn get_random_values_ref(this: &Crypto, buf: &mut [u8]) -> Result<(), JsValue>;
+    #[wasm_bindgen(thread_local_v2, static_string)]
+    static SHARED_ARRAY_BUFFER_NAME: JsString = "SharedArrayBuffer";
 }