diff --git a/docshell/test/navigation/file_reload_large_postdata.sjs b/docshell/test/navigation/file_reload_large_postdata.sjs
new file mode 100644
index 000000000000..a2644179081a
--- /dev/null
+++ b/docshell/test/navigation/file_reload_large_postdata.sjs
@@ -0,0 +1,46 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80 ft=javascript: */
+"use strict";
+
+const BinaryInputStream = CC(
+  "@mozilla.org/binaryinputstream;1",
+  "nsIBinaryInputStream",
+  "setInputStream"
+);
+
+Cu.importGlobalProperties(["URLSearchParams"]);
+
+function readStream(inputStream) {
+  let available = 0;
+  let result = [];
+  while ((available = inputStream.available()) > 0) {
+    result.push(inputStream.readBytes(available));
+  }
+  return result.join("");
+}
+
+function handleRequest(request, response) {
+  let rv = (() => {
+    try {
+      if (request.method != "POST") {
+        return "ERROR: not a POST request";
+      }
+
+      let body = new URLSearchParams(
+        readStream(new BinaryInputStream(request.bodyInputStream))
+      );
+      return body.get("payload").length;
+    } catch (e) {
+      return "ERROR: Exception: " + e;
+    }
+  })();
+
+  response.setHeader("Content-Type", "text/html", false);
+  response.setHeader("Cache-Control", "no-cache", false);
+  response.write(`<!DOCTYPE HTML>
+<script>
+let rv = (${JSON.stringify(rv)});
+opener.postMessage(rv, "*");
+</script>
+  `);
+}
diff --git a/docshell/test/navigation/mochitest.ini b/docshell/test/navigation/mochitest.ini
index 693121ca10ca..18d6a233a660 100644
--- a/docshell/test/navigation/mochitest.ini
+++ b/docshell/test/navigation/mochitest.ini
@@ -119,3 +119,6 @@ skip-if = true # This was disabled for a few years now anyway, bug 1677544
 [test_triggeringprincipal_iframe_iframe_window_open.html]
 [test_contentpolicy_block_window.html]
 [test_rate_limit_location_change.html]
+[test_reload_large_postdata.html]
+support-files =
+  file_reload_large_postdata.sjs
diff --git a/docshell/test/navigation/test_reload_large_postdata.html b/docshell/test/navigation/test_reload_large_postdata.html
new file mode 100644
index 000000000000..767316cd5f6f
--- /dev/null
+++ b/docshell/test/navigation/test_reload_large_postdata.html
@@ -0,0 +1,58 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <script src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
+</head>
+<body>
+<p id="display"></p>
+
+<form id="form" action="file_reload_large_postdata.sjs" target="_blank" rel="opener" method="POST">
+  <input id="input" name="payload" type="hidden" value=""/>
+</form>
+
+<pre id="test">
+<script>
+// This is derived from `kTooLargeStream` in `IPCStreamUtils.cpp`.
+const kTooLargeStream = 1024 * 1024;
+
+function waitForPopup(expected) {
+  return new Promise(resolve => {
+    addEventListener("message", evt => {
+      info("got message!");
+      is(evt.source.opener, window, "the event source's opener should be this window");
+      is(evt.data, expected, "got the expected data from the popup");
+      resolve(evt.source);
+    }, { once: true });
+  });
+}
+
+add_task(async function() {
+  await SpecialPowers.pushPrefEnv({"set": [["dom.confirm_repost.testing.always_accept", true]]});
+  let form = document.getElementById("form");
+  let input = document.getElementById("input");
+
+  // Create a very large value to include in the post payload. This should
+  // ensure that the value isn't sent directly over IPC, and is instead sent as
+  // an async inputstream.
+  let payloadSize = kTooLargeStream;
+
+  let popupReady = waitForPopup(payloadSize);
+  input.value = "A".repeat(payloadSize);
+  form.submit();
+
+  let popup = await popupReady;
+  try {
+    let popupReady2 = waitForPopup(payloadSize);
+    info("reloading popup");
+    popup.location.reload();
+    let popup2 = await popupReady2;
+    is(popup, popup2);
+  } finally {
+    popup.close();
+  }
+});
+</script>
+</pre>
+</body>
+</html>
diff --git a/ipc/glue/IPCStreamUtils.cpp b/ipc/glue/IPCStreamUtils.cpp
index fba98891fe2e..6d3e1868168e 100644
--- a/ipc/glue/IPCStreamUtils.cpp
+++ b/ipc/glue/IPCStreamUtils.cpp
@@ -40,6 +40,8 @@ bool SerializeInputStreamWithFdsChild(nsIIPCSerializableInputStream* aStream,
   MOZ_RELEASE_ASSERT(aStream);
   MOZ_ASSERT(aManager);
 
+  // If you change this size, please also update the payload size in
+  // test_reload_large_postdata.html.
   const uint64_t kTooLargeStream = 1024 * 1024;
 
   uint32_t sizeUsed = 0;
