Well, for a torrent to stay healthy, users have to seed after download, so it can't possibly have the same UX as a standard browser file download unless you want to either kill the ecosystem or hide from the user what is consuming upload bandwidth.

That said, for large files, I much prefer the UX of a well-designed torrent client like Transmission to my web browser. If nothing else, the downloads are reliably resumable.

> Is there a fully in-browser torrent option that has the same UX as a regular file download in Firefox?

Opera did back in the day.