Webpage with txt saved with LF Linux end-of-line on Windows
To reproduce: use https://www.ietf.org/rfc/rfc0821.txt and click Open Menu in the upper right-hand corner and select Save Page (as text) into a destination folder and filename.
Then use Notepad++ to open the file. It shows the lines end with LF when selecting View->Show Symbols->Show All characters on the Notepad++ menu.
How do I tell Firefox to stop doing this and use use CRLF?
Wšykne wótegrona (1)
The file you link is really LF terminated (unix file) and not a CRLF file (DOS file), so when you download it, you get what the server is sending. firefox should not translate files, as this might be unwanted and be classified as "file corruption" in many situations.
You can always convert the file type using notepad++ on windows or dos2unix and unix2dos on linux. of course, there are more tools for doing this, just search