The server reports MIME type
text/plain in the HTTP
Content-Type header field and Firefox diligently interprets and displays the file content as such. The server behaviour is a known bug in Launchpad itself.
You can circumvent the issue
- with the entries "Save as…" or "Copy link address"¹ in the context menu of the download link,
- by dragging the link text to a writeable directory in a file manager, that supports HTTP resources, like Nautilus.
A side note concerning the reported behaviour of Chrome/Chromium: web browsers shouldn't care about file extensions to determine file types according to RFC 1341. They should only care about the MIME string sent in the HTTP response header. If a browser disregards that string in favour of file extensions or content type sniffing, they may be doing something useful for the user in most situations, but they're violating RFC 1341 in the process.
¹ Plus an HTTP client, that doesn't try to display file content, like
curl, or DownThemAll!.