When does Firefox display a download progress bar?

I’m downloading a file from a server that is returning a Content-Length header, but Firefox does not show the download progress bar.

What are the criteria that Firefox uses to decide when to display a progress bar on downloads? I’m not familiar with the Firefox source code but if anybody has a link to the relevant code, I’d appreciate it.

Here are my response headers:

  HTTP/1.1 200 OK
  Server: nginx/1.18.0 (Ubuntu)
  Date: Thu, 03 Jan 2023 19:53:33 GMT
  Content-Type: application/octet-stream
  Content-Length: 1255777601
  Connection: keep-alive
  content-disposition: attachment; filename="contents.mkv"
  last-modified: Tue, 03 Jan 2023 05:23:58 GMT
  x-frame-options: deny
  accept-ranges: bytes
  x-xss-protection: 1; mode=block
  x-robots-tag: noindex, nofollow
  etag: "3e8d6d3abe32c1cb55ede5ec36a5d7b911703e8cef8398b6c779e2c5a26f1b58"
  referrer-policy: no-referrer
  content-language: en-US
  x-content-type-options: nosniff
  Strict-Transport-Security: max-age=86400