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diff --git a/packages/meshbay-hub/src/meshbay_hub/static/sw.js b/packages/meshbay-hub/src/meshbay_hub/static/sw.js
new file mode 100644
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+++ b/packages/meshbay-hub/src/meshbay_hub/static/sw.js
@@ -0,0 +1,60 @@
+/**
+ * Service worker: the only way to stream a download to disk in Firefox.
+ *
+ * Chrome and Edge have the File System Access API — the page opens a file and
+ * writes to it. Firefox and Safari do not, and the alternative there was to
+ * collect the whole download in memory and hand the browser a blob, which is
+ * not an option for a file measured in gigabytes.
+ *
+ * So the page makes up a URL, tells this worker what stream answers it, and
+ * navigates a hidden iframe there. The worker replies with the stream and a
+ * Content-Disposition header, and the browser does what it does with any
+ * download: writes it to disk as it arrives, showing its own progress, with
+ * nothing buffered in the tab.
+ *
+ * It caches nothing and intercepts nothing else. Every request that is not one
+ * of these downloads falls through untouched.
+ */
+
+const PREFIX = '/_mbdl/';
+const pending = new Map();
+
+self.addEventListener('install', () => self.skipWaiting());
+self.addEventListener('activate', (event) => event.waitUntil(self.clients.claim()));
+
+self.addEventListener('message', (event) => {
+  const data = event.data || {};
+  if (data.type !== 'mbdl' || !data.id || !data.readable) return;
+  pending.set(data.id, {
+    readable: data.readable,
+    filename: data.filename || 'download',
+    size: Number(data.size) || 0,
+  });
+  // A tab that is closed before it navigates would leave a stream here for the
+  // life of the worker.
+  setTimeout(() => pending.delete(data.id), 60000);
+});
+
+self.addEventListener('fetch', (event) => {
+  const url = new URL(event.request.url);
+  if (url.origin !== self.location.origin || !url.pathname.startsWith(PREFIX)) {
+    return;   // not ours — the network handles it
+  }
+
+  const entry = pending.get(url.pathname.slice(PREFIX.length));
+  if (!entry) return;
+  pending.delete(url.pathname.slice(PREFIX.length));
+
+  const headers = {
+    'Content-Type': 'application/octet-stream',
+    // filename* so a name with accents or spaces survives the trip.
+    'Content-Disposition':
+      `attachment; filename*=UTF-8''${encodeURIComponent(entry.filename)}`,
+    'Cache-Control': 'no-store',
+  };
+  // Only when it is known. A zip is assembled as it goes and announcing a
+  // length we then miss would truncate the file.
+  if (entry.size > 0) headers['Content-Length'] = String(entry.size);
+
+  event.respondWith(new Response(entry.readable, { headers }));
+});