diff --git a/extension/src/service-worker/index.ts b/extension/src/service-worker/index.ts
index 1d6b703..c3fc4c7 100644
--- a/extension/src/service-worker/index.ts
+++ b/extension/src/service-worker/index.ts
@@ -1,4 +1,7 @@
-/// Service worker entry point for the idfoto Chrome extension.
+/// Background script entry point for the idfoto browser extension.
+///
+/// In Chrome this runs as a service worker (MV3). In Firefox this runs
+/// as a persistent background script. WASM loading adapts automatically.
 ///
 /// Loads the WASM module, manages vault state (master key, manifest, git host),
 /// and routes all messages from the popup and content scripts.
@@ -22,14 +25,14 @@ const totpSecretCache: Map<string, string> = new Map();
 
 // --- WASM initialization ---
 
-// Chrome MV3 service workers do NOT support dynamic import().
-// Instead we load the WASM binary via fetch() and use the synchronous
-// initSync() function exported by wasm-pack's --target web output.
-// The JS glue is imported statically so webpack bundles it into
-// the service worker script.
+// Chrome MV3 uses service workers which do NOT support dynamic import().
+// Firefox MV3 uses background scripts which DO support dynamic import().
+// We detect the environment at runtime and use the appropriate loading strategy.
 
 // @ts-ignore TS2307 — resolved by webpack alias / copy
-import initSync, * as wasmBindings from '../../wasm/idfoto_wasm.js';
+import initDefault, { initSync } from '../../wasm/idfoto_wasm.js';
+// @ts-ignore TS2307
+import * as wasmBindings from '../../wasm/idfoto_wasm.js';
 
 type WasmModule = typeof wasmBindings;
 let wasm: WasmModule | null = null;
@@ -37,10 +40,21 @@ let wasm: WasmModule | null = null;
 async function initWasm(): Promise<WasmModule> {
   if (wasm) return wasm;
 
-  // Fetch the .wasm binary and instantiate synchronously
-  const wasmResponse = await fetch(chrome.runtime.getURL('idfoto_wasm_bg.wasm'));
-  const wasmBytes = await wasmResponse.arrayBuffer();
-  initSync({ module: new WebAssembly.Module(wasmBytes) });
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  const SWGlobalScope = (globalThis as any).ServiceWorkerGlobalScope as (new () => ServiceWorker) | undefined;
+  const isServiceWorker = typeof SWGlobalScope !== 'undefined'
+    && self instanceof (SWGlobalScope as unknown as typeof EventTarget);
+
+  if (isServiceWorker) {
+    // Chrome: fetch WASM binary and instantiate synchronously
+    const wasmResponse = await fetch(chrome.runtime.getURL('idfoto_wasm_bg.wasm'));
+    const wasmBytes = await wasmResponse.arrayBuffer();
+    initSync({ module: new WebAssembly.Module(wasmBytes) });
+  } else {
+    // Firefox: background script — async init works
+    const wasmUrl = chrome.runtime.getURL('idfoto_wasm_bg.wasm');
+    await initDefault(wasmUrl);
+  }
 
   vault.setWasm(wasmBindings);
   wasm = wasmBindings;