Firefox is weird.
- Copy from Firefox, paste in Firefox → as reported.
- Copy from Chrome, paste in Firefox → no newlines at all.
- Copy from Chrome, paste in Notepad, copy again and paste in Firefox → works.
- Copy from Firefox and paste in Chrome → works too.
My guess is that both browsers put richtext into the clipboard, and Firefox has problems with pasting that into a plaintext input field. Or could it be that Discourse somehow makes FF think that the editor supports richtext?