diff --git a/Makefile b/Makefile
index b5266c5..994cbd2 100644
--- a/Makefile
+++ b/Makefile
@@ -21,6 +21,10 @@
 $(BUILD)/firefox: $(SRC)
 	mkdir -p $@
 	cp -r $(SRC) $@/
+	# Firefox MV3 has no background.service_worker / type:"module":
+	# bundle api.js + background.js into one classic script and use event-page scripts.
+	node tools/bundle-background.js $@
+	node -e "const fs=require('fs'); const p='$@/manifest.json'; const m=JSON.parse(fs.readFileSync(p)); m.background={scripts:['src/background.js']}; fs.writeFileSync(p, JSON.stringify(m,null,2));"
 	sed -i 's|url("../assets/|url("assets/|g' $@/lib/gnexus-ui-kit.css
 	sed -i "s|url('/assets/|url('assets/|g" $@/lib/gnexus-ui-kit.css
 
diff --git a/README.md b/README.md
index b25b851..802694b 100644
--- a/README.md
+++ b/README.md
@@ -19,9 +19,13 @@
 
 ### Firefox
 
-1. Откройте `about:debugging#/runtime/this-firefox`.
-2. Нажмите "Load Temporary Add-on" и выберите `manifest.json`.
-3. (Или установите `.zip` из `dist/`).
+> Firefox MV3 не поддерживает `background.service_worker`, поэтому грузить нужно
+> собранную версию из `build/firefox/` (или `.zip` из `dist/`), а не корневой `manifest.json`.
+
+1. Соберите: `make firefox`.
+2. Откройте `about:debugging#/runtime/this-firefox`.
+3. Нажмите "Load Temporary Add-on" и выберите `build/firefox/manifest.json`
+   (или установите `dist/gnexus-creds-extension-firefox-0.1.0.zip`).
 
 ## Настройка
 
diff --git a/tools/bundle-background.js b/tools/bundle-background.js
new file mode 100644
index 0000000..996e55e
--- /dev/null
+++ b/tools/bundle-background.js
@@ -0,0 +1,31 @@
+#!/usr/bin/env node
+/**
+ * Bundle src/api.js + src/background.js into a single classic (non-module)
+ * background.js for Firefox MV3, which does not support background.service_worker
+ * or type:"module" (event-page scripts run as classic scripts sharing one scope).
+ *
+ * Usage: node tools/bundle-background.js <build-dir>
+ */
+const fs = require("fs");
+const path = require("path");
+
+const dir = process.argv[2];
+if (!dir) {
+  console.error("build dir argument required");
+  process.exit(1);
+}
+
+const apiPath = path.join(dir, "src", "api.js");
+const bgPath = path.join(dir, "src", "background.js");
+
+// Strip ES module keywords so the result is valid as a classic script.
+const api = fs.readFileSync(apiPath, "utf8").replace(/^export /gm, "");
+const bg = fs.readFileSync(bgPath, "utf8").replace(
+  /^\s*import\s+.*\s+from\s+["']\.\/api\.js["'];\s*\n/m,
+  ""
+);
+
+// api.js function declarations are hoisted into the shared classic scope,
+// so background.js can reference them without an import.
+fs.writeFileSync(bgPath, api.trimEnd() + "\n\n" + bg);
+fs.rmSync(apiPath);
\ No newline at end of file