Forum der Gemeinschaft Firefox

Just recently when logged onto Hotcopper.com.au have been getting a message that reads "This page is slowing Firefox down. To speed up this browser, stop this page." There is a STOP button, that helps for a moment or two only. When I open the program with MS Edge, the problem does nor occur. I was going to remove the Firefox App but saw the repair option. Repairing did not stop the problem. Any suggestions? John I have a near new Acer laptop running Win 11.

Summary: Uploading a file to a custom webapp with Chromium (Edge) works but consistently fails in Firefox. In Firefox the request is created, and times out. The server receives no file and logs “missing file.” The same webapp code is running on both Chromium and Firefox.

Environment:

• Next.js app (pages/api route)
• Endpoint: POST /api/file-upload
• Client: fetch(url, { method: "POST", body: FormData, credentials: "include" })
• Server: export const config = { api: { bodyParser: false } } + multipart parsing (formidable)
• Edge/Chromium: Version 141.0.3537.85
• Firefox Version 147.0.3 (64-bit)
• OS: Ubuntu 22 / Linux

Steps to reproduce:

1. Create a FormData object and append a local file:

  const fd = new FormData();
  fd.append("template", file, file.name);
  fetch(`/api/file-upload`, {
    method: "POST",
    body: fd,

1. In Edge/Chrome, upload succeeds. Server receives multipart body and parses the file. Actual behavior in Firefox:

• Transferred: 0 B
• No response status/body
• Client times out (Upload times out)
• File upload request received
• No bytes read server‑side.

Evidence:

• Edge request logs:
  • content-type: multipart/form-data; boundary=----WebKitFormBoundary...
  • bytesRead ~1.69MB
• Firefox request logs:
  • Transferred 0 B
  • File upload missing file field

Notes:

• Client FormData is valid; we log file.name and file.size right before fetch.
• There is a CSP report‑only warning (frame‑ancestors) when using a fallback iframe approach, but we are not using iframe in the failing case.

Question: Is there a known issue where Firefox blocks or fails to send and multipart/body can be stripped before it leaves the browser? Is there a recommended workaround that does not rely on browser sniffing?

Moderator note: This question has been edited to correct the formatting.

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