That default_newlines pref is no longer supported (112+) and its behavior defaults to a DIV. You now need to use Shift + Enter to get a new line (<br>).

OK. I understand that the second variable has been deprecated, but why does it remain in about:config?

Can you speak to the first variable settings? Are there values that can be set that will accomplish the goal of having firefox go to a new line on a single enter key press and enter a blank line and go to a new line on a double enter key press?

This pref is only in about:config because you modified it from the default value and in such a case it is stored in prefs,js in the profile folder and not removed automatically. You can see that it has a trashcan at the far right indicating that this is a user created pref that possibly isn't supported and you can click this icon to remove this pref.

Thanks, Cor-el.

Still no comment on the first variable?

Here is the explanation of the first preference from the source code. Doesn't seem relevant to your issue:

# How line breakers are treated in single line editor:
# * 0: Only remove the leading and trailing newlines.
# * 1: Remove the first newline and all characters following it.
# * 2: Replace newlines with spaces (default of Firefox).
# * 3: Remove newlines from the string.
# * 4: Replace newlines with commas (default of Thunderbird).
# * 5: Collapse newlines and surrounding white space characters and
#      remove them from the string.
# Other values are treated as 1.
- name: editor.singleLine.pasteNewlines
  type: int32_t
  value: 2
  mirror: always

As its name says, editor.singleLine.pasteNewlines is only about pasting text that possibly contains newline characters in a single line input field. A multi-line text area does support newline characters, but a single line doesn't and this pref sets how to behave in this case. This has nothing to do with typing text in a multi-line text area and how to interpret the Enter key.